7 Abstract Operations

These operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other, more specialized abstract operations are defined throughout this specification.

7.1 Type Conversion

The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. The conversion abstract operations are polymorphic; they can accept a value of any ECMAScript language type. But no other specification types are used with these operations.

The BigInt type has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.

7.1.1 ToPrimitive ( input [ , preferredType ] )

The abstract operation ToPrimitive takes argument input (an ECMAScript language value) and optional argument preferredType (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. It converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint preferredType to favour that type. It performs the following steps when called:

  1. If input is an Object, then
    1. Let exoticToPrim be ? GetMethod(input, %Symbol.toPrimitive%).
    2. If exoticToPrim is not undefined, then
      1. If preferredType is not present, then
        1. Let hint be "default".
      2. Else if preferredType is string, then
        1. Let hint be "string".
      3. Else,
        1. Assert: preferredType is number.
        2. Let hint be "number".
      4. Let result be ? Call(exoticToPrim, input, « hint »).
      5. If result is not an Object, return result.
      6. Throw a TypeError exception.
    3. If preferredType is not present, let preferredType be number.
    4. Return ? OrdinaryToPrimitive(input, preferredType).
  2. Return input.
 Note

When ToPrimitive is called without a hint, then it generally behaves as if the hint were number. However, objects may over-ride this behaviour by defining a %Symbol.toPrimitive% method. Of the objects defined in this specification only Dates (see 21.4.4.45) and Symbol objects (see 20.4.3.5) over-ride the default ToPrimitive behaviour. Dates treat the absence of a hint as if the hint were string.

7.1.1.1 OrdinaryToPrimitive ( O, hint )

The abstract operation OrdinaryToPrimitive takes arguments O (an Object) and hint (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. If hint is string, then
    1. Let methodNames be « "toString", "valueOf" ».
  2. Else,
    1. Let methodNames be « "valueOf", "toString" ».
  3. For each element name of methodNames, do
    1. Let method be ? Get(O, name).
    2. If IsCallable(method) is true, then
      1. Let result be ? Call(method, O).
      2. If result is not an Object, return result.
  4. Throw a TypeError exception.

7.1.2 ToBoolean ( argument )

The abstract operation ToBoolean takes argument argument (an ECMAScript language value) and returns a Boolean. It converts argument to a value of type Boolean. It performs the following steps when called:

  1. If argument is a Boolean, return argument.
  2. If argument is one of undefined, null, +0𝔽, -0𝔽, NaN, 0ℤ, or the empty String, return false.
  3. NOTE: This step is replaced in section B.3.6.1.
  4. Return true.

7.1.3 ToNumeric ( value )

The abstract operation ToNumeric takes argument value (an ECMAScript language value) and returns either a normal completion containing either a Number or a BigInt, or a throw completion. It returns value converted to a Number or a BigInt. It performs the following steps when called:

  1. Let primValue be ? ToPrimitive(value, number).
  2. If primValue is a BigInt, return primValue.
  3. Return ? ToNumber(primValue).

7.1.4 ToNumber ( argument )

The abstract operation ToNumber takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Number or a throw completion. It converts argument to a value of type Number. It performs the following steps when called:

  1. If argument is a Number, return argument.
  2. If argument is either a Symbol or a BigInt, throw a TypeError exception.
  3. If argument is undefined, return NaN.
  4. If argument is either null or false, return +0𝔽.
  5. If argument is true, return 1𝔽.
  6. If argument is a String, return StringToNumber(argument).
  7. Assert: argument is an Object.
  8. Let primValue be ? ToPrimitive(argument, number).
  9. Assert: primValue is not an Object.
  10. Return ? ToNumber(primValue).

7.1.4.1 ToNumber Applied to the String Type

The abstract operation StringToNumber specifies how to convert a String value to a Number value, using the following grammar.

Syntax

StringNumericLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt StrWhiteSpace ::: StrWhiteSpaceChar StrWhiteSpaceopt StrWhiteSpaceChar ::: WhiteSpace LineTerminator StrNumericLiteral ::: StrDecimalLiteral NonDecimalIntegerLiteral[~Sep] StrDecimalLiteral ::: StrUnsignedDecimalLiteral + StrUnsignedDecimalLiteral - StrUnsignedDecimalLiteral StrUnsignedDecimalLiteral ::: Infinity DecimalDigits[~Sep] . DecimalDigits[~Sep]opt ExponentPart[~Sep]opt . DecimalDigits[~Sep] ExponentPart[~Sep]opt DecimalDigits[~Sep] ExponentPart[~Sep]opt

All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (12.9.3)

Note

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral:

7.1.4.1.1 StringToNumber ( str )

The abstract operation StringToNumber takes argument str (a String) and returns a Number. It performs the following steps when called:

  1. Let literal be ParseText(str, StringNumericLiteral).
  2. If literal is a List of errors, return NaN.
  3. Return the StringNumericValue of literal.

7.1.4.1.2 Runtime Semantics: StringNumericValue

The syntax-directed operation StringNumericValue takes no arguments and returns a Number.

Note

The conversion of a StringNumericLiteral to a Number value is similar overall to the determination of the NumericValue of a NumericLiteral (see 12.9.3), but some of the details are different.

It is defined piecewise over the following productions:

StringNumericLiteral ::: StrWhiteSpaceopt
  1. Return +0𝔽.
 StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
  1. Return the StringNumericValue of StrNumericLiteral.
 StrNumericLiteral ::: NonDecimalIntegerLiteral
  1. Return 𝔽(MV of NonDecimalIntegerLiteral).
 StrDecimalLiteral ::: - StrUnsignedDecimalLiteral
  1. Let a be the StringNumericValue of StrUnsignedDecimalLiteral.
  2. If a is +0𝔽, return -0𝔽.
  3. Return -a.
 StrUnsignedDecimalLiteral ::: Infinity
  1. Return +∞𝔽.
 StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigitsopt ExponentPartopt
  1. Let a be the MV of the first DecimalDigits.
  2. If the second DecimalDigits is present, then
    1. Let b be the MV of the second DecimalDigits.
    2. Let n be the number of code points in the second DecimalDigits.
  3. Else,
    1. Let b be 0.
    2. Let n be 0.
  4. If ExponentPart is present, let e be the MV of ExponentPart. Otherwise, let e be 0.
  5. Return RoundMVResult((a + (b × 10-n)) × 10e).
 StrUnsignedDecimalLiteral ::: . DecimalDigits ExponentPartopt
  1. Let b be the MV of DecimalDigits.
  2. If ExponentPart is present, let e be the MV of ExponentPart. Otherwise, let e be 0.
  3. Let n be the number of code points in DecimalDigits.
  4. Return RoundMVResult(b × 10e - n).
 StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPartopt
  1. Let a be the MV of DecimalDigits.
  2. If ExponentPart is present, let e be the MV of ExponentPart. Otherwise, let e be 0.
  3. Return RoundMVResult(a × 10e).

7.1.4.1.3 RoundMVResult ( n )

The abstract operation RoundMVResult takes argument n (a mathematical value) and returns a Number. It converts n to a Number in an implementation-defined manner. For the purposes of this abstract operation, a digit is significant if it is not zero or there is a non-zero digit to its left and there is a non-zero digit to its right. For the purposes of this abstract operation, "the mathematical value denoted by" a representation of a mathematical value is the inverse of "the decimal representation of" a mathematical value. It performs the following steps when called:

  1. If the decimal representation of n has 20 or fewer significant digits, return 𝔽(n).
  2. Let option1 be the mathematical value denoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit.
  3. Let option2 be the mathematical value denoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit and then incrementing it at the 20th position (with carrying as necessary).
  4. Let chosen be an implementation-defined choice of either option1 or option2.
  5. Return 𝔽(chosen).

7.1.5 ToIntegerOrInfinity ( argument )

The abstract operation ToIntegerOrInfinity takes argument argument (an ECMAScript language value) and returns either a normal completion containing either an integer, +∞, or -∞, or a throw completion. It converts argument to an integer representing its Number value with fractional part truncated, or to +∞ or -∞ when that Number value is infinite. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is one of NaN, +0𝔽, or -0𝔽, return 0.
  3. If number is +∞𝔽, return +∞.
  4. If number is -∞𝔽, return -∞.
  5. Return truncate(ℝ(number)).
 Note
𝔽(ToIntegerOrInfinity(x)) never returns -0𝔽 for any value of x. The truncation of the fractional part is performed after converting x to a mathematical value.

7.1.6 ToInt32 ( argument )

The abstract operation ToInt32 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 232 integral Number values in the inclusive interval from 𝔽(-231) to 𝔽(231 - 1). It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int32bit be int modulo 232.
  5. If int32bit ≥ 231, return 𝔽(int32bit - 232); otherwise return 𝔽(int32bit).
 Note

Given the above definition of ToInt32:

  • The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
  • ToInt32(ToUint32(x)) is the same value as ToInt32(x) for all values of x. (It is to preserve this latter property that +∞𝔽 and -∞𝔽 are mapped to +0𝔽.)
  • ToInt32 maps -0𝔽 to +0𝔽.

7.1.7 ToUint32 ( argument )

The abstract operation ToUint32 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 232 integral Number values in the inclusive interval from +0𝔽 to 𝔽(232 - 1). It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int32bit be int modulo 232.
  5. Return 𝔽(int32bit).
 Note

Given the above definition of ToUint32:

  • Step 5 is the only difference between ToUint32 and ToInt32.
  • The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
  • ToUint32(ToInt32(x)) is the same value as ToUint32(x) for all values of x. (It is to preserve this latter property that +∞𝔽 and -∞𝔽 are mapped to +0𝔽.)
  • ToUint32 maps -0𝔽 to +0𝔽.

7.1.8 ToInt16 ( argument )

The abstract operation ToInt16 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 216 integral Number values in the inclusive interval from 𝔽(-215) to 𝔽(215 - 1). It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int16bit be int modulo 216.
  5. If int16bit ≥ 215, return 𝔽(int16bit - 216); otherwise return 𝔽(int16bit).

7.1.9 ToUint16 ( argument )

The abstract operation ToUint16 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 216 integral Number values in the inclusive interval from +0𝔽 to 𝔽(216 - 1). It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int16bit be int modulo 216.
  5. Return 𝔽(int16bit).
 Note

Given the above definition of ToUint16:

  • The substitution of 216 for 232 in step 4 is the only difference between ToUint32 and ToUint16.
  • ToUint16 maps -0𝔽 to +0𝔽.

7.1.10 ToInt8 ( argument )

The abstract operation ToInt8 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 28 integral Number values in the inclusive interval from -128𝔽 to 127𝔽. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int8bit be int modulo 28.
  5. If int8bit ≥ 27, return 𝔽(int8bit - 28); otherwise return 𝔽(int8bit).

7.1.11 ToUint8 ( argument )

The abstract operation ToUint8 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 28 integral Number values in the inclusive interval from +0𝔽 to 255𝔽. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int8bit be int modulo 28.
  5. Return 𝔽(int8bit).

7.1.12 ToUint8Clamp ( argument )

The abstract operation ToUint8Clamp takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It clamps and rounds argument to one of 28 integral Number values in the inclusive interval from +0𝔽 to 255𝔽. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is NaN, return +0𝔽.
  3. Let mv be the extended mathematical value of number.
  4. Let clamped be the result of clamping mv between 0 and 255.
  5. Let f be floor(clamped).
  6. If clamped < f + 0.5, return 𝔽(f).
  7. If clamped > f + 0.5, return 𝔽(f + 1).
  8. If f is even, return 𝔽(f). Otherwise, return 𝔽(f + 1).
 Note

Unlike most other ECMAScript integer conversion operations, ToUint8Clamp rounds rather than truncates non-integral values. It also uses “round half to even” tie-breaking, which differs from the “round half up” tie-breaking of Math.round.

7.1.13 ToBigInt ( argument )

The abstract operation ToBigInt takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to a BigInt value, or throws if an implicit conversion from Number would be required. It performs the following steps when called:

  1. Let prim be ? ToPrimitive(argument, number).
  2. Return the value that prim corresponds to in Table 12.
Table 12: BigInt Conversions
Argument Type Result
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return 1n if prim is true and 0n if prim is false.
BigInt Return prim.
Number Throw a TypeError exception.
String
  1. Let n be StringToBigInt(prim).
  2. If n is undefined, throw a SyntaxError exception.
  3. Return n.
Symbol Throw a TypeError exception.

7.1.14 StringToBigInt ( str )

The abstract operation StringToBigInt takes argument str (a String) and returns a BigInt or undefined. It performs the following steps when called:

  1. Let literal be ParseText(str, StringIntegerLiteral).
  2. If literal is a List of errors, return undefined.
  3. Let mv be the MV of literal.
  4. Assert: mv is an integer.
  5. Return ℤ(mv).

7.1.14.1 StringIntegerLiteral Grammar

StringToBigInt uses the following grammar.

Syntax

StringIntegerLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrIntegerLiteral StrWhiteSpaceopt StrIntegerLiteral ::: SignedInteger[~Sep] NonDecimalIntegerLiteral[~Sep]

7.1.14.2 Runtime Semantics: MV

7.1.15 ToBigInt64 ( argument )

The abstract operation ToBigInt64 takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to one of 264 BigInt values in the inclusive interval from ℤ(-263) to ℤ(263 - 1). It performs the following steps when called:

  1. Let n be ? ToBigInt(argument).
  2. Let int64bit be ℝ(n) modulo 264.
  3. If int64bit ≥ 263, return ℤ(int64bit - 264); otherwise return ℤ(int64bit).

7.1.16 ToBigUint64 ( argument )

The abstract operation ToBigUint64 takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to one of 264 BigInt values in the inclusive interval from 0ℤ to ℤ(264 - 1). It performs the following steps when called:

  1. Let n be ? ToBigInt(argument).
  2. Let int64bit be ℝ(n) modulo 264.
  3. Return ℤ(int64bit).

7.1.17 ToString ( argument )

The abstract operation ToString takes argument argument (an ECMAScript language value) and returns either a normal completion containing a String or a throw completion. It converts argument to a value of type String. It performs the following steps when called:

  1. If argument is a String, return argument.
  2. If argument is a Symbol, throw a TypeError exception.
  3. If argument is undefined, return "undefined".
  4. If argument is null, return "null".
  5. If argument is true, return "true".
  6. If argument is false, return "false".
  7. If argument is a Number, return Number::toString(argument, 10).
  8. If argument is a BigInt, return BigInt::toString(argument, 10).
  9. Assert: argument is an Object.
  10. Let primValue be ? ToPrimitive(argument, string).
  11. Assert: primValue is not an Object.
  12. Return ? ToString(primValue).

7.1.18 ToObject ( argument )

The abstract operation ToObject takes argument argument (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It converts argument to a value of type Object according to Table 13:

Table 13: ToObject Conversions
Argument Type Result
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return a new Boolean object whose [[BooleanData]] internal slot is set to argument. See 20.3 for a description of Boolean objects.
Number Return a new Number object whose [[NumberData]] internal slot is set to argument. See 21.1 for a description of Number objects.
String Return a new String object whose [[StringData]] internal slot is set to argument. See 22.1 for a description of String objects.
Symbol Return a new Symbol object whose [[SymbolData]] internal slot is set to argument. See 20.4 for a description of Symbol objects.
BigInt Return a new BigInt object whose [[BigIntData]] internal slot is set to argument. See 21.2 for a description of BigInt objects.
Object Return argument.

7.1.19 ToPropertyKey ( argument )

The abstract operation ToPropertyKey takes argument argument (an ECMAScript language value) and returns either a normal completion containing a property key or a throw completion. It converts argument to a value that can be used as a property key. It performs the following steps when called:

  1. Let key be ? ToPrimitive(argument, string).
  2. If key is a Symbol, then
    1. Return key.
  3. Return ! ToString(key).

7.1.20 ToLength ( argument )

The abstract operation ToLength takes argument argument (an ECMAScript language value) and returns either a normal completion containing a non-negative integral Number or a throw completion. It clamps and truncates argument to a non-negative integral Number suitable for use as the length of an array-like object. It performs the following steps when called:

  1. Let len be ? ToIntegerOrInfinity(argument).
  2. If len ≤ 0, return +0𝔽.
  3. Return 𝔽(min(len, 253 - 1)).

7.1.21 CanonicalNumericIndexString ( argument )

The abstract operation CanonicalNumericIndexString takes argument argument (a String) and returns a Number or undefined. If argument is either "-0" or exactly matches ToString(n) for some Number value n, it returns the respective Number value. Otherwise, it returns undefined. It performs the following steps when called:

  1. If argument is "-0", return -0𝔽.
  2. Let n be ! ToNumber(argument).
  3. If ! ToString(n) is argument, return n.
  4. Return undefined.

A canonical numeric string is any String value for which the CanonicalNumericIndexString abstract operation does not return undefined.

7.1.22 ToIndex ( value )

The abstract operation ToIndex takes argument value (an ECMAScript language value) and returns either a normal completion containing a non-negative integer or a throw completion. It converts value to an integer and returns that integer if it is non-negative and corresponds with an integer index. Otherwise, it throws an exception. It performs the following steps when called:

  1. Let integer be ? ToIntegerOrInfinity(value).
  2. If integer is not in the inclusive interval from 0 to 253 - 1, throw a RangeError exception.
  3. Return integer.

7.2 Testing and Comparison Operations

7.2.1 RequireObjectCoercible ( argument )

The abstract operation RequireObjectCoercible takes argument argument (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It throws an error if argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 14:

Table 14: RequireObjectCoercible Results
Argument Type Result
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return argument.
Number Return argument.
String Return argument.
Symbol Return argument.
BigInt Return argument.
Object Return argument.

7.2.2 IsArray ( argument )

The abstract operation IsArray takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If argument is not an Object, return false.
  2. If argument is an Array exotic object, return true.
  3. If argument is a Proxy exotic object, then
    1. Perform ? ValidateNonRevokedProxy(argument).
    2. Let proxyTarget be argument.[[ProxyTarget]].
    3. Return ? IsArray(proxyTarget).
  4. Return false.

7.2.3 IsCallable ( argument )

The abstract operation IsCallable takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a callable function with a [[Call]] internal method. It performs the following steps when called:

  1. If argument is not an Object, return false.
  2. If argument has a [[Call]] internal method, return true.
  3. Return false.

7.2.4 IsConstructor ( argument )

The abstract operation IsConstructor takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a function object with a [[Construct]] internal method. It performs the following steps when called:

  1. If argument is not an Object, return false.
  2. If argument has a [[Construct]] internal method, return true.
  3. Return false.

7.2.5 IsExtensible ( O )

The abstract operation IsExtensible takes argument O (an Object) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether additional properties can be added to O. It performs the following steps when called:

  1. Return ? O.[[IsExtensible]]().

7.2.6 IsRegExp ( argument )

The abstract operation IsRegExp takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If argument is not an Object, return false.
  2. Let matcher be ? Get(argument, %Symbol.match%).
  3. If matcher is not undefined, return ToBoolean(matcher).
  4. If argument has a [[RegExpMatcher]] internal slot, return true.
  5. Return false.

7.2.7 Static Semantics: IsStringWellFormedUnicode ( string )

The abstract operation IsStringWellFormedUnicode takes argument string (a String) and returns a Boolean. It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and determines whether it is a well formed UTF-16 sequence. It performs the following steps when called:

  1. Let len be the length of string.
  2. Let k be 0.
  3. Repeat, while k < len,
    1. Let cp be CodePointAt(string, k).
    2. If cp.[[IsUnpairedSurrogate]] is true, return false.
    3. Set k to k + cp.[[CodeUnitCount]].
  4. Return true.

7.2.8 SameType ( x, y )

The abstract operation SameType takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same type. It performs the following steps when called:

  1. If x is undefined and y is undefined, return true.
  2. If x is null and y is null, return true.
  3. If x is a Boolean and y is a Boolean, return true.
  4. If x is a Number and y is a Number, return true.
  5. If x is a BigInt and y is a BigInt, return true.
  6. If x is a Symbol and y is a Symbol, return true.
  7. If x is a String and y is a String, return true.
  8. If x is an Object and y is an Object, return true.
  9. Return false.

7.2.9 SameValue ( x, y )

The abstract operation SameValue takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same value. It performs the following steps when called:

  1. If SameType(x, y) is false, return false.
  2. If x is a Number, then
    1. Return Number::sameValue(x, y).
  3. Return SameValueNonNumber(x, y).
 Note

This algorithm differs from the IsStrictlyEqual Algorithm by treating all NaN values as equivalent and by differentiating +0𝔽 from -0𝔽.

7.2.10 SameValueZero ( x, y )

The abstract operation SameValueZero takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same value (ignoring the difference between +0𝔽 and -0𝔽). It performs the following steps when called:

  1. If SameType(x, y) is false, return false.
  2. If x is a Number, then
    1. Return Number::sameValueZero(x, y).
  3. Return SameValueNonNumber(x, y).
 Note

SameValueZero differs from SameValue only in that it treats +0𝔽 and -0𝔽 as equivalent.

7.2.11 SameValueNonNumber ( x, y )

The abstract operation SameValueNonNumber takes arguments x (an ECMAScript language value, but not a Number) and y (an ECMAScript language value, but not a Number) and returns a Boolean. It performs the following steps when called:

  1. Assert: SameType(x, y) is true.
  2. If x is either null or undefined, return true.
  3. If x is a BigInt, then
    1. Return BigInt::equal(x, y).
  4. If x is a String, then
    1. If x and y have the same length and the same code units in the same positions, return true; otherwise, return false.
  5. If x is a Boolean, then
    1. If x and y are both true or both false, return true; otherwise, return false.
  6. NOTE: All other ECMAScript language values are compared by identity.
  7. If x is y, return true; otherwise, return false.
 Note 1
For expository purposes, some cases are handled separately within this algorithm even if it is unnecessary to do so.
Note 2
The specifics of what "x is y" means are detailed in 5.2.7.

7.2.12 IsLessThan ( x, y, LeftFirst )

The abstract operation IsLessThan takes arguments x (an ECMAScript language value), y (an ECMAScript language value), and LeftFirst (a Boolean) and returns either a normal completion containing either a Boolean or undefined, or a throw completion. It provides the semantics for the comparison x < y, returning true, false, or undefined (which indicates that at least one operand is NaN). The LeftFirst flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. If LeftFirst is true, the x parameter corresponds to an expression that occurs to the left of the y parameter's corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. It performs the following steps when called:

  1. If LeftFirst is true, then
    1. Let px be ? ToPrimitive(x, number).
    2. Let py be ? ToPrimitive(y, number).
  2. Else,
    1. NOTE: The order of evaluation needs to be reversed to preserve left to right evaluation.
    2. Let py be ? ToPrimitive(y, number).
    3. Let px be ? ToPrimitive(x, number).
  3. If px is a String and py is a String, then
    1. Let lx be the length of px.
    2. Let ly be the length of py.
    3. For each integer i such that 0 ≤ i < min(lx, ly), in ascending order, do
      1. Let cx be the numeric value of the code unit at index i within px.
      2. Let cy be the numeric value of the code unit at index i within py.
      3. If cx < cy, return true.
      4. If cx > cy, return false.
    4. If lx < ly, return true. Otherwise, return false.
  4. Else,
    1. If px is a BigInt and py is a String, then
      1. Let ny be StringToBigInt(py).
      2. If ny is undefined, return undefined.
      3. Return BigInt::lessThan(px, ny).
    2. If px is a String and py is a BigInt, then
      1. Let nx be StringToBigInt(px).
      2. If nx is undefined, return undefined.
      3. Return BigInt::lessThan(nx, py).
    3. NOTE: Because px and py are primitive values, evaluation order is not important.
    4. Let nx be ? ToNumeric(px).
    5. Let ny be ? ToNumeric(py).
    6. If SameType(nx, ny) is true, then
      1. If nx is a Number, then
        1. Return Number::lessThan(nx, ny).
      2. Else,
        1. Assert: nx is a BigInt.
        2. Return BigInt::lessThan(nx, ny).
    7. Assert: nx is a BigInt and ny is a Number, or nx is a Number and ny is a BigInt.
    8. If nx or ny is NaN, return undefined.
    9. If nx is -∞𝔽 or ny is +∞𝔽, return true.
    10. If nx is +∞𝔽 or ny is -∞𝔽, return false.
    11. If ℝ(nx) < ℝ(ny), return true; otherwise return false.
 Note 1

Step 3 differs from step 1.c in the algorithm that handles the addition operator + (13.15.3) by using the logical-and operation instead of the logical-or operation.

 Note 2

The comparison of Strings uses a simple lexicographic ordering on sequences of UTF-16 code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode Standard but not in the same normalization form could test as unequal. Also note that lexicographic ordering by code unit differs from ordering by code point for Strings containing surrogate pairs.

7.2.13 IsLooselyEqual ( x, y )

The abstract operation IsLooselyEqual takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It provides the semantics for the == operator. It performs the following steps when called:

  1. If SameType(x, y) is true, then
    1. Return IsStrictlyEqual(x, y).
  2. If x is null and y is undefined, return true.
  3. If x is undefined and y is null, return true.
  4. NOTE: This step is replaced in section B.3.6.2.
  5. If x is a Number and y is a String, return ! IsLooselyEqual(x, ! ToNumber(y)).
  6. If x is a String and y is a Number, return ! IsLooselyEqual(! ToNumber(x), y).
  7. If x is a BigInt and y is a String, then
    1. Let n be StringToBigInt(y).
    2. If n is undefined, return false.
    3. Return ! IsLooselyEqual(x, n).
  8. If x is a String and y is a BigInt, return ! IsLooselyEqual(y, x).
  9. If x is a Boolean, return ! IsLooselyEqual(! ToNumber(x), y).
  10. If y is a Boolean, return ! IsLooselyEqual(x, ! ToNumber(y)).
  11. If x is either a String, a Number, a BigInt, or a Symbol and y is an Object, return ! IsLooselyEqual(x, ? ToPrimitive(y)).
  12. If x is an Object and y is either a String, a Number, a BigInt, or a Symbol, return ! IsLooselyEqual(? ToPrimitive(x), y).
  13. If x is a BigInt and y is a Number, or if x is a Number and y is a BigInt, then
    1. If x is not finite or y is not finite, return false.
    2. If ℝ(x) = ℝ(y), return true; otherwise return false.
  14. Return false.

7.2.14 IsStrictlyEqual ( x, y )

The abstract operation IsStrictlyEqual takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It provides the semantics for the === operator. It performs the following steps when called:

  1. If SameType(x, y) is false, return false.
  2. If x is a Number, then
    1. Return Number::equal(x, y).
  3. Return SameValueNonNumber(x, y).
 Note

This algorithm differs from the SameValue Algorithm in its treatment of signed zeroes and NaNs.

7.3 Operations on Objects

7.3.1 MakeBasicObject ( internalSlotsList )

The abstract operation MakeBasicObject takes argument internalSlotsList (a List of internal slot names) and returns an Object. It is the source of all ECMAScript objects that are created algorithmically, including both ordinary objects and exotic objects. It factors out common steps used in creating all objects, and centralizes object creation. It performs the following steps when called:

  1. Set internalSlotsList to the list-concatenation of internalSlotsList and « [[PrivateElements]] ».
  2. Let obj be a newly created object with an internal slot for each name in internalSlotsList.
  3. Set obj.[[PrivateElements]] to a new empty List.
  4. Set obj's essential internal methods to the default ordinary object definitions specified in 10.1.
  5. Assert: If the caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList contains [[Prototype]].
  6. Assert: If the caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential internal methods, then internalSlotsList contains [[Extensible]].
  7. If internalSlotsList contains [[Extensible]], set obj.[[Extensible]] to true.
  8. Return obj.
 Note

Within this specification, exotic objects are created in abstract operations such as ArrayCreate and BoundFunctionCreate by first calling MakeBasicObject to obtain a basic, foundational object, and then overriding some or all of that object's internal methods. In order to encapsulate exotic object creation, the object's essential internal methods are never modified outside those operations.

7.3.2 Get ( O, P )

The abstract operation Get takes arguments O (an Object) and P (a property key) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to retrieve the value of a specific property of an object. It performs the following steps when called:

  1. Return ? O.[[Get]](P, O).

7.3.3 GetV ( V, P )

The abstract operation GetV takes arguments V (an ECMAScript language value) and P (a property key) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to retrieve the value of a specific property of an ECMAScript language value. If the value is not an object, the property lookup is performed using a wrapper object appropriate for the type of the value. It performs the following steps when called:

  1. Let O be ? ToObject(V).
  2. Return ? O.[[Get]](P, V).

7.3.4 Set ( O, P, V, Throw )

The abstract operation Set takes arguments O (an Object), P (a property key), V (an ECMAScript language value), and Throw (a Boolean) and returns either a normal completion containing unused or a throw completion. It is used to set the value of a specific property of an object. V is the new value for the property. It performs the following steps when called:

  1. Let success be ? O.[[Set]](P, V, O).
  2. If success is false and Throw is true, throw a TypeError exception.
  3. Return unused.

7.3.5 CreateDataProperty ( O, P, V )

The abstract operation CreateDataProperty takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It is used to create a new own property of an object. It performs the following steps when called:

  1. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
  2. Return ? O.[[DefineOwnProperty]](P, newDesc).
 Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.

7.3.6 CreateDataPropertyOrThrow ( O, P, V )

The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It is used to create a new own property of an object. It throws a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:

  1. Let success be ? CreateDataProperty(O, P, V).
  2. If success is false, throw a TypeError exception.
  3. Return unused.
 Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false causing this operation to throw a TypeError exception.

7.3.7 CreateNonEnumerableDataPropertyOrThrow ( O, P, V )

The abstract operation CreateNonEnumerableDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns unused. It is used to create a new non-enumerable own property of an ordinary object. It performs the following steps when called:

  1. Assert: O is an ordinary, extensible object with no non-configurable properties.
  2. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
  3. Perform ! DefinePropertyOrThrow(O, P, newDesc).
  4. Return unused.
 Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator except it is not enumerable. Normally, the property will not already exist. If it does exist, DefinePropertyOrThrow is guaranteed to complete normally.

7.3.8 DefinePropertyOrThrow ( O, P, desc )

The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a property key), and desc (a Property Descriptor) and returns either a normal completion containing unused or a throw completion. It is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:

  1. Let success be ? O.[[DefineOwnProperty]](P, desc).
  2. If success is false, throw a TypeError exception.
  3. Return unused.

7.3.9 DeletePropertyOrThrow ( O, P )

The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a property key) and returns either a normal completion containing unused or a throw completion. It is used to remove a specific own property of an object. It throws an exception if the property is not configurable. It performs the following steps when called:

  1. Let success be ? O.[[Delete]](P).
  2. If success is false, throw a TypeError exception.
  3. Return unused.

7.3.10 GetMethod ( V, P )

The abstract operation GetMethod takes arguments V (an ECMAScript language value) and P (a property key) and returns either a normal completion containing either a function object or undefined, or a throw completion. It is used to get the value of a specific property of an ECMAScript language value when the value of the property is expected to be a function. It performs the following steps when called:

  1. Let func be ? GetV(V, P).
  2. If func is either undefined or null, return undefined.
  3. If IsCallable(func) is false, throw a TypeError exception.
  4. Return func.

7.3.11 HasProperty ( O, P )

The abstract operation HasProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether an object has a property with the specified property key. The property may be either own or inherited. It performs the following steps when called:

  1. Return ? O.[[HasProperty]](P).

7.3.12 HasOwnProperty ( O, P )

The abstract operation HasOwnProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether an object has an own property with the specified property key. It performs the following steps when called:

  1. Let desc be ? O.[[GetOwnProperty]](P).
  2. If desc is undefined, return false.
  3. Return true.

7.3.13 Call ( F, V [ , argumentsList ] )

The abstract operation Call takes arguments F (an ECMAScript language value) and V (an ECMAScript language value) and optional argument argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to call the [[Call]] internal method of a function object. F is the function object, V is an ECMAScript language value that is the this value of the [[Call]], and argumentsList is the value passed to the corresponding argument of the internal method. If argumentsList is not present, a new empty List is used as its value. It performs the following steps when called:

  1. If argumentsList is not present, set argumentsList to a new empty List.
  2. If IsCallable(F) is false, throw a TypeError exception.
  3. Return ? F.[[Call]](V, argumentsList).

7.3.14 Construct ( F [ , argumentsList [ , newTarget ] ] )

The abstract operation Construct takes argument F (a constructor) and optional arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It is used to call the [[Construct]] internal method of a function object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, a new empty List is used as its value. If newTarget is not present, F is used as its value. It performs the following steps when called:

  1. If newTarget is not present, set newTarget to F.
  2. If argumentsList is not present, set argumentsList to a new empty List.
  3. Return ? F.[[Construct]](argumentsList, newTarget).
 Note

If newTarget is not present, this operation is equivalent to: new F(...argumentsList)

7.3.15 SetIntegrityLevel ( O, level )

The abstract operation SetIntegrityLevel takes arguments O (an Object) and level (sealed or frozen) and returns either a normal completion containing a Boolean or a throw completion. It is used to fix the set of own properties of an object. It performs the following steps when called:

  1. Let status be ? O.[[PreventExtensions]]().
  2. If status is false, return false.
  3. Let keys be ? O.[[OwnPropertyKeys]]().
  4. If level is sealed, then
    1. For each element k of keys, do
      1. Perform ? DefinePropertyOrThrow(O, k, PropertyDescriptor { [[Configurable]]: false }).
  5. Else,
    1. Assert: level is frozen.
    2. For each element k of keys, do
      1. Let currentDesc be ? O.[[GetOwnProperty]](k).
      2. If currentDesc is not undefined, then
        1. If IsAccessorDescriptor(currentDesc) is true, then
          1. Let desc be the PropertyDescriptor { [[Configurable]]: false }.
        2. Else,
          1. Let desc be the PropertyDescriptor { [[Configurable]]: false, [[Writable]]: false }.
        3. Perform ? DefinePropertyOrThrow(O, k, desc).
  6. Return true.

7.3.16 TestIntegrityLevel ( O, level )

The abstract operation TestIntegrityLevel takes arguments O (an Object) and level (sealed or frozen) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine if the set of own properties of an object are fixed. It performs the following steps when called:

  1. Let extensible be ? IsExtensible(O).
  2. If extensible is true, return false.
  3. NOTE: If the object is extensible, none of its properties are examined.
  4. Let keys be ? O.[[OwnPropertyKeys]]().
  5. For each element k of keys, do
    1. Let currentDesc be ? O.[[GetOwnProperty]](k).
    2. If currentDesc is not undefined, then
      1. If currentDesc.[[Configurable]] is true, return false.
      2. If level is frozen and IsDataDescriptor(currentDesc) is true, then
        1. If currentDesc.[[Writable]] is true, return false.
  6. Return true.

7.3.17 CreateArrayFromList ( elements )

The abstract operation CreateArrayFromList takes argument elements (a List of ECMAScript language values) and returns an Array. It is used to create an Array whose elements are provided by elements. It performs the following steps when called:

  1. Let array be ! ArrayCreate(0).
  2. Let n be 0.
  3. For each element e of elements, do
    1. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(n)), e).
    2. Set n to n + 1.
  4. Return array.

7.3.18 LengthOfArrayLike ( obj )

The abstract operation LengthOfArrayLike takes argument obj (an Object) and returns either a normal completion containing a non-negative integer or a throw completion. It returns the value of the "length" property of an array-like object. It performs the following steps when called:

  1. Return ℝ(? ToLength(? Get(obj, "length"))).

An array-like object is any object for which this operation returns a normal completion.

Note 1
Typically, an array-like object would also have some properties with integer index names. However, that is not a requirement of this definition.
Note 2
Arrays and String objects are examples of array-like objects.

7.3.19 CreateListFromArrayLike ( obj [ , validElementTypes ] )

The abstract operation CreateListFromArrayLike takes argument obj (an ECMAScript language value) and optional argument validElementTypes (all or property-key) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It is used to create a List value whose elements are provided by the indexed properties of obj. elementTypes contains the names of ECMAScript Language Types that are allowed for element values of the List that is created. It performs the following steps when called:

  1. If validElementTypes is not present, set validElementTypes to all.
  2. If obj is not an Object, throw a TypeError exception.
  3. Let len be ? LengthOfArrayLike(obj).
  4. Let list be a new empty List.
  5. Let index be 0.
  6. Repeat, while index < len,
    1. Let indexName be ! ToString(𝔽(index)).
    2. Let next be ? Get(obj, indexName).
    3. If validElementTypes is property-key and next is not a property key, throw a TypeError exception.
    4. Append next to list.
    5. Set index to index + 1.
  7. Return list.

7.3.20 Invoke ( V, P [ , argumentsList ] )

The abstract operation Invoke takes arguments V (an ECMAScript language value) and P (a property key) and optional argument argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to call a method property of an ECMAScript language value. V serves as both the lookup point for the property and the this value of the call. argumentsList is the list of arguments values passed to the method. If argumentsList is not present, a new empty List is used as its value. It performs the following steps when called:

  1. If argumentsList is not present, set argumentsList to a new empty List.
  2. Let func be ? GetV(V, P).
  3. Return ? Call(func, V, argumentsList).

7.3.21 OrdinaryHasInstance ( C, O )

The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript language value) and O (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It implements the default algorithm for determining if O inherits from the instance object inheritance path provided by C. It performs the following steps when called:

  1. If IsCallable(C) is false, return false.
  2. If C has a [[BoundTargetFunction]] internal slot, then
    1. Let BC be C.[[BoundTargetFunction]].
    2. Return ? InstanceofOperator(O, BC).
  3. If O is not an Object, return false.
  4. Let P be ? Get(C, "prototype").
  5. If P is not an Object, throw a TypeError exception.
  6. Repeat,
    1. Set O to ? O.[[GetPrototypeOf]]().
    2. If O is null, return false.
    3. If SameValue(P, O) is true, return true.

7.3.22 SpeciesConstructor ( O, defaultConstructor )

The abstract operation SpeciesConstructor takes arguments O (an Object) and defaultConstructor (a constructor) and returns either a normal completion containing a constructor or a throw completion. It is used to retrieve the constructor that should be used to create new objects that are derived from O. defaultConstructor is the constructor to use if a constructor %Symbol.species% property cannot be found starting from O. It performs the following steps when called:

  1. Let C be ? Get(O, "constructor").
  2. If C is undefined, return defaultConstructor.
  3. If C is not an Object, throw a TypeError exception.
  4. Let S be ? Get(C, %Symbol.species%).
  5. If S is either undefined or null, return defaultConstructor.
  6. If IsConstructor(S) is true, return S.
  7. Throw a TypeError exception.

7.3.23 EnumerableOwnProperties ( O, kind )

The abstract operation EnumerableOwnProperties takes arguments O (an Object) and kind (key, value, or key+value) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It performs the following steps when called:

  1. Let ownKeys be ? O.[[OwnPropertyKeys]]().
  2. Let results be a new empty List.
  3. For each element key of ownKeys, do
    1. If key is a String, then
      1. Let desc be ? O.[[GetOwnProperty]](key).
      2. If desc is not undefined and desc.[[Enumerable]] is true, then
        1. If kind is key, then
          1. Append key to results.
        2. Else,
          1. Let value be ? Get(O, key).
          2. If kind is value, then
            1. Append value to results.
          3. Else,
            1. Assert: kind is key+value.
            2. Let entry be CreateArrayFromList(« key, value »).
            3. Append entry to results.
  4. Return results.

7.3.24 GetFunctionRealm ( obj )

The abstract operation GetFunctionRealm takes argument obj (a function object) and returns either a normal completion containing a Realm Record or a throw completion. It performs the following steps when called:

  1. If obj has a [[Realm]] internal slot, then
    1. Return obj.[[Realm]].
  2. If obj is a bound function exotic object, then
    1. Let boundTargetFunction be obj.[[BoundTargetFunction]].
    2. Return ? GetFunctionRealm(boundTargetFunction).
  3. If obj is a Proxy exotic object, then
    1. Perform ? ValidateNonRevokedProxy(obj).
    2. Let proxyTarget be obj.[[ProxyTarget]].
    3. Return ? GetFunctionRealm(proxyTarget).
  4. Return the current Realm Record.
 Note

Step 4 will only be reached if obj is a non-standard function exotic object that does not have a [[Realm]] internal slot.

7.3.25 CopyDataProperties ( target, source, excludedItems )

The abstract operation CopyDataProperties takes arguments target (an Object), source (an ECMAScript language value), and excludedItems (a List of property keys) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. If source is either undefined or null, return unused.
  2. Let from be ! ToObject(source).
  3. Let keys be ? from.[[OwnPropertyKeys]]().
  4. For each element nextKey of keys, do
    1. Let excluded be false.
    2. For each element e of excludedItems, do
      1. If SameValue(e, nextKey) is true, then
        1. Set excluded to true.
    3. If excluded is false, then
      1. Let desc be ? from.[[GetOwnProperty]](nextKey).
      2. If desc is not undefined and desc.[[Enumerable]] is true, then
        1. Let propValue be ? Get(from, nextKey).
        2. Perform ! CreateDataPropertyOrThrow(target, nextKey, propValue).
  5. Return unused.
 Note

The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.

7.3.26 PrivateElementFind ( O, P )

The abstract operation PrivateElementFind takes arguments O (an Object) and P (a Private Name) and returns a PrivateElement or empty. It performs the following steps when called:

  1. If O.[[PrivateElements]] contains a PrivateElement pe such that pe.[[Key]] is P, then
    1. Return pe.
  2. Return empty.

7.3.27 PrivateFieldAdd ( O, P, value )

The abstract operation PrivateFieldAdd takes arguments O (an Object), P (a Private Name), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. If the host is a web browser, then
    1. Perform ? HostEnsureCanAddPrivateElement(O).
  2. Let entry be PrivateElementFind(O, P).
  3. If entry is not empty, throw a TypeError exception.
  4. Append PrivateElement { [[Key]]: P, [[Kind]]: field, [[Value]]: value } to O.[[PrivateElements]].
  5. Return unused.

7.3.28 PrivateMethodOrAccessorAdd ( O, method )

The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object) and method (a PrivateElement) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Assert: method.[[Kind]] is either method or accessor.
  2. If the host is a web browser, then
    1. Perform ? HostEnsureCanAddPrivateElement(O).
  3. Let entry be PrivateElementFind(O, method.[[Key]]).
  4. If entry is not empty, throw a TypeError exception.
  5. Append method to O.[[PrivateElements]].
  6. Return unused.
 Note

The values for private methods and accessors are shared across instances. This operation does not create a new copy of the method or accessor.

7.3.29 HostEnsureCanAddPrivateElement ( O )

The host-defined abstract operation HostEnsureCanAddPrivateElement takes argument O (an Object) and returns either a normal completion containing unused or a throw completion. It allows host environments to prevent the addition of private elements to particular host-defined exotic objects.

An implementation of HostEnsureCanAddPrivateElement must conform to the following requirements:

The default implementation of HostEnsureCanAddPrivateElement is to return NormalCompletion(unused).

This abstract operation is only invoked by ECMAScript hosts that are web browsers.

7.3.30 PrivateGet ( O, P )

The abstract operation PrivateGet takes arguments O (an Object) and P (a Private Name) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let entry be PrivateElementFind(O, P).
  2. If entry is empty, throw a TypeError exception.
  3. If entry.[[Kind]] is either field or method, then
    1. Return entry.[[Value]].
  4. Assert: entry.[[Kind]] is accessor.
  5. If entry.[[Get]] is undefined, throw a TypeError exception.
  6. Let getter be entry.[[Get]].
  7. Return ? Call(getter, O).

7.3.31 PrivateSet ( O, P, value )

The abstract operation PrivateSet takes arguments O (an Object), P (a Private Name), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let entry be PrivateElementFind(O, P).
  2. If entry is empty, throw a TypeError exception.
  3. If entry.[[Kind]] is field, then
    1. Set entry.[[Value]] to value.
  4. Else if entry.[[Kind]] is method, then
    1. Throw a TypeError exception.
  5. Else,
    1. Assert: entry.[[Kind]] is accessor.
    2. If entry.[[Set]] is undefined, throw a TypeError exception.
    3. Let setter be entry.[[Set]].
    4. Perform ? Call(setter, O, « value »).
  6. Return unused.

7.3.32 DefineField ( receiver, fieldRecord )

The abstract operation DefineField takes arguments receiver (an Object) and fieldRecord (a ClassFieldDefinition Record) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let fieldName be fieldRecord.[[Name]].
  2. Let initializer be fieldRecord.[[Initializer]].
  3. If initializer is not empty, then
    1. Let initValue be ? Call(initializer, receiver).
  4. Else,
    1. Let initValue be undefined.
  5. If fieldName is a Private Name, then
    1. Perform ? PrivateFieldAdd(receiver, fieldName, initValue).
  6. Else,
    1. Assert: fieldName is a property key.
    2. Perform ? CreateDataPropertyOrThrow(receiver, fieldName, initValue).
  7. Return unused.

7.3.33 InitializeInstanceElements ( O, constructor )

The abstract operation InitializeInstanceElements takes arguments O (an Object) and constructor (an ECMAScript function object) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let methods be the value of constructor.[[PrivateMethods]].
  2. For each PrivateElement method of methods, do
    1. Perform ? PrivateMethodOrAccessorAdd(O, method).
  3. Let fields be the value of constructor.[[Fields]].
  4. For each element fieldRecord of fields, do
    1. Perform ? DefineField(O, fieldRecord).
  5. Return unused.

7.3.34 AddValueToKeyedGroup ( groups, key, value )

The abstract operation AddValueToKeyedGroup takes arguments groups (a List of Records with fields [[Key]] (an ECMAScript language value) and [[Elements]] (a List of ECMAScript language values)), key (an ECMAScript language value), and value (an ECMAScript language value) and returns unused. It performs the following steps when called:

  1. For each Record { [[Key]], [[Elements]] } g of groups, do
    1. If SameValue(g.[[Key]], key) is true, then
      1. Assert: Exactly one element of groups meets this criterion.
      2. Append value to g.[[Elements]].
      3. Return unused.
  2. Let group be the Record { [[Key]]: key, [[Elements]]: « value » }.
  3. Append group to groups.
  4. Return unused.

7.3.35 GroupBy ( items, callback, keyCoercion )

The abstract operation GroupBy takes arguments items (an ECMAScript language value), callback (an ECMAScript language value), and keyCoercion (property or collection) and returns either a normal completion containing a List of Records with fields [[Key]] (an ECMAScript language value) and [[Elements]] (a List of ECMAScript language values), or a throw completion. It performs the following steps when called:

  1. Perform ? RequireObjectCoercible(items).
  2. If IsCallable(callback) is false, throw a TypeError exception.
  3. Let groups be a new empty List.
  4. Let iteratorRecord be ? GetIterator(items, sync).
  5. Let k be 0.
  6. Repeat,
    1. If k ≥ 253 - 1, then
      1. Let error be ThrowCompletion(a newly created TypeError object).
      2. Return ? IteratorClose(iteratorRecord, error).
    2. Let next be ? IteratorStepValue(iteratorRecord).
    3. If next is done, then
      1. Return groups.
    4. Let value be next.
    5. Let key be Completion(Call(callback, undefined, « value, 𝔽(k) »)).
    6. IfAbruptCloseIterator(key, iteratorRecord).
    7. If keyCoercion is property, then
      1. Set key to Completion(ToPropertyKey(key)).
      2. IfAbruptCloseIterator(key, iteratorRecord).
    8. Else,
      1. Assert: keyCoercion is collection.
      2. Set key to CanonicalizeKeyedCollectionKey(key).
    9. Perform AddValueToKeyedGroup(groups, key, value).
    10. Set k to k + 1.

7.3.36 SetterThatIgnoresPrototypeProperties ( thisValue, home, p, v )

The abstract operation SetterThatIgnoresPrototypeProperties takes arguments thisValue (an ECMAScript language value), home (an Object), p (a property key), and v (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. If thisValue is not an Object, then
    1. Throw a TypeError exception.
  2. If SameValue(thisValue, home) is true, then
    1. NOTE: Throwing here emulates assignment to a non-writable data property on the home object in strict mode code.
    2. Throw a TypeError exception.
  3. Let desc be ? thisValue.[[GetOwnProperty]](p).
  4. If desc is undefined, then
    1. Perform ? CreateDataPropertyOrThrow(thisValue, p, v).
  5. Else,
    1. Perform ? Set(thisValue, p, v, true).
  6. Return unused.

7.4 Operations on Iterator Objects

See Common Iteration Interfaces (27.1).

7.4.1 Iterator Records

An Iterator Record is a Record value used to encapsulate an Iterator or AsyncIterator along with the next method.

Iterator Records have the fields listed in Table 15.

Table 15: Iterator Record Fields
Field Name Value Meaning
[[Iterator]] an Object An object that conforms to the Iterator or AsyncIterator interface.
[[NextMethod]] an ECMAScript language value The next method of the [[Iterator]] object.
[[Done]] a Boolean Whether the iterator has completed or been closed.

7.4.2 GetIteratorDirect ( obj )

The abstract operation GetIteratorDirect takes argument obj (an Object) and returns either a normal completion containing an Iterator Record or a throw completion. It performs the following steps when called:

  1. Let nextMethod be ? Get(obj, "next").
  2. Let iteratorRecord be the Iterator Record { [[Iterator]]: obj, [[NextMethod]]: nextMethod, [[Done]]: false }.
  3. Return iteratorRecord.

7.4.3 GetIteratorFromMethod ( obj, method )

The abstract operation GetIteratorFromMethod takes arguments obj (an ECMAScript language value) and method (a function object) and returns either a normal completion containing an Iterator Record or a throw completion. It performs the following steps when called:

  1. Let iterator be ? Call(method, obj).
  2. If iterator is not an Object, throw a TypeError exception.
  3. Return ? GetIteratorDirect(iterator).

7.4.4 GetIterator ( obj, kind )

The abstract operation GetIterator takes arguments obj (an ECMAScript language value) and kind (sync or async) and returns either a normal completion containing an Iterator Record or a throw completion. It performs the following steps when called:

  1. If kind is async, then
    1. Let method be ? GetMethod(obj, %Symbol.asyncIterator%).
    2. If method is undefined, then
      1. Let syncMethod be ? GetMethod(obj, %Symbol.iterator%).
      2. If syncMethod is undefined, throw a TypeError exception.
      3. Let syncIteratorRecord be ? GetIteratorFromMethod(obj, syncMethod).
      4. Return CreateAsyncFromSyncIterator(syncIteratorRecord).
  2. Else,
    1. Let method be ? GetMethod(obj, %Symbol.iterator%).
  3. If method is undefined, throw a TypeError exception.
  4. Return ? GetIteratorFromMethod(obj, method).

7.4.5 GetIteratorFlattenable ( obj, primitiveHandling )

The abstract operation GetIteratorFlattenable takes arguments obj (an ECMAScript language value) and primitiveHandling (iterate-string-primitives or reject-primitives) and returns either a normal completion containing an Iterator Record or a throw completion. It performs the following steps when called:

  1. If obj is not an Object, then
    1. If primitiveHandling is reject-primitives, throw a TypeError exception.
    2. Assert: primitiveHandling is iterate-string-primitives.
    3. If obj is not a String, throw a TypeError exception.
  2. Let method be ? GetMethod(obj, %Symbol.iterator%).
  3. If method is undefined, then
    1. Let iterator be obj.
  4. Else,
    1. Let iterator be ? Call(method, obj).
  5. If iterator is not an Object, throw a TypeError exception.
  6. Return ? GetIteratorDirect(iterator).

7.4.6 IteratorNext ( iteratorRecord [ , value ] )

The abstract operation IteratorNext takes argument iteratorRecord (an Iterator Record) and optional argument value (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. If value is not present, then
    1. Let result be Completion(Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]])).
  2. Else,
    1. Let result be Completion(Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « value »)).
  3. If result is a throw completion, then
    1. Set iteratorRecord.[[Done]] to true.
    2. Return ? result.
  4. Set result to ! result.
  5. If result is not an Object, then
    1. Set iteratorRecord.[[Done]] to true.
    2. Throw a TypeError exception.
  6. Return result.

7.4.7 IteratorComplete ( iteratorResult )

The abstract operation IteratorComplete takes argument iteratorResult (an Object) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Return ToBoolean(? Get(iteratorResult, "done")).

7.4.8 IteratorValue ( iteratorResult )

The abstract operation IteratorValue takes argument iteratorResult (an Object) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Return ? Get(iteratorResult, "value").

7.4.9 IteratorStep ( iteratorRecord )

The abstract operation IteratorStep takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing either an Object or done, or a throw completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either done indicating that the iterator has reached its end or the IteratorResult object if a next value is available. It performs the following steps when called:

  1. Let result be ? IteratorNext(iteratorRecord).
  2. Let done be Completion(IteratorComplete(result)).
  3. If done is a throw completion, then
    1. Set iteratorRecord.[[Done]] to true.
    2. Return ? done.
  4. Set done to ! done.
  5. If done is true, then
    1. Set iteratorRecord.[[Done]] to true.
    2. Return done.
  6. Return result.

7.4.10 IteratorStepValue ( iteratorRecord )

The abstract operation IteratorStepValue takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing either an ECMAScript language value or done, or a throw completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either done indicating that the iterator has reached its end or the value from the IteratorResult object if a next value is available. It performs the following steps when called:

  1. Let result be ? IteratorStep(iteratorRecord).
  2. If result is done, then
    1. Return done.
  3. Let value be Completion(IteratorValue(result)).
  4. If value is a throw completion, then
    1. Set iteratorRecord.[[Done]] to true.
  5. Return ? value.

7.4.11 IteratorClose ( iteratorRecord, completion )

The abstract operation IteratorClose takes arguments iteratorRecord (an Iterator Record) and completion (a Completion Record) and returns a Completion Record. It is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

  1. Assert: iteratorRecord.[[Iterator]] is an Object.
  2. Let iterator be iteratorRecord.[[Iterator]].
  3. Let innerResult be Completion(GetMethod(iterator, "return")).
  4. If innerResult is a normal completion, then
    1. Let return be innerResult.[[Value]].
    2. If return is undefined, return ? completion.
    3. Set innerResult to Completion(Call(return, iterator)).
  5. If completion is a throw completion, return ? completion.
  6. If innerResult is a throw completion, return ? innerResult.
  7. If innerResult.[[Value]] is not an Object, throw a TypeError exception.
  8. Return ? completion.

7.4.12 IfAbruptCloseIterator ( value, iteratorRecord )

IfAbruptCloseIterator is a shorthand for a sequence of algorithm steps that use an Iterator Record. An algorithm step of the form:

  1. IfAbruptCloseIterator(value, iteratorRecord).

means the same thing as:

  1. Assert: value is a Completion Record.
  2. If value is an abrupt completion, return ? IteratorClose(iteratorRecord, value).
  3. Else, set value to ! value.

7.4.13 AsyncIteratorClose ( iteratorRecord, completion )

The abstract operation AsyncIteratorClose takes arguments iteratorRecord (an Iterator Record) and completion (a Completion Record) and returns a Completion Record. It is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

  1. Assert: iteratorRecord.[[Iterator]] is an Object.
  2. Let iterator be iteratorRecord.[[Iterator]].
  3. Let innerResult be Completion(GetMethod(iterator, "return")).
  4. If innerResult is a normal completion, then
    1. Let return be innerResult.[[Value]].
    2. If return is undefined, return ? completion.
    3. Set innerResult to Completion(Call(return, iterator)).
    4. If innerResult is a normal completion, set innerResult to Completion(Await(innerResult.[[Value]])).
  5. If completion is a throw completion, return ? completion.
  6. If innerResult is a throw completion, return ? innerResult.
  7. If innerResult.[[Value]] is not an Object, throw a TypeError exception.
  8. Return ? completion.

7.4.14 CreateIteratorResultObject ( value, done )

The abstract operation CreateIteratorResultObject takes arguments value (an ECMAScript language value) and done (a Boolean) and returns an Object that conforms to the IteratorResult interface. It creates an object that conforms to the IteratorResult interface. It performs the following steps when called:

  1. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  2. Perform ! CreateDataPropertyOrThrow(obj, "value", value).
  3. Perform ! CreateDataPropertyOrThrow(obj, "done", done).
  4. Return obj.

7.4.15 CreateListIteratorRecord ( list )

The abstract operation CreateListIteratorRecord takes argument list (a List of ECMAScript language values) and returns an Iterator Record. It creates an Iterator (27.1.1.2) object record whose next method returns the successive elements of list. It performs the following steps when called:

  1. Let closure be a new Abstract Closure with no parameters that captures list and performs the following steps when called:
    1. For each element E of list, do
      1. Perform ? GeneratorYield(CreateIteratorResultObject(E, false)).
    2. Return NormalCompletion(undefined).
  2. Let iterator be CreateIteratorFromClosure(closure, empty, %Iterator.prototype%).
  3. Return the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]: %GeneratorFunction.prototype.prototype.next%, [[Done]]: false }.
 Note

The list iterator object is never directly accessible to ECMAScript code.

7.4.16 IteratorToList ( iteratorRecord )

The abstract operation IteratorToList takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It performs the following steps when called:

  1. Let values be a new empty List.
  2. Repeat,
    1. Let next be ? IteratorStepValue(iteratorRecord).
    2. If next is done, then
      1. Return values.
    3. Append next to values.