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ES6 Concetps

The document discusses various JavaScript concepts including the 'this' operator, closures, synchronous and asynchronous programming, callbacks, callback hell, promises, and async/await. It explains how closures allow for private variables, the difference between synchronous and asynchronous execution, and the challenges of managing nested callbacks. Additionally, it covers the use of promises to handle asynchronous operations and introduces IIFE (Immediately Invoked Function Expression).

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Asma Eman
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3 views9 pages

ES6 Concetps

The document discusses various JavaScript concepts including the 'this' operator, closures, synchronous and asynchronous programming, callbacks, callback hell, promises, and async/await. It explains how closures allow for private variables, the difference between synchronous and asynchronous execution, and the challenges of managing nested callbacks. Additionally, it covers the use of promises to handle asynchronous operations and introduces IIFE (Immediately Invoked Function Expression).

Uploaded by

Asma Eman
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
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‘this’ operator in JS

Closure in JS
A function is defined inside another function, and the inner function has access to the
variables and scope of the outer function. Allow for private variables and state
maintenance.

Sync in JS
Synchronous
Synchronous means the code runs in a particular sequence of instructions given in the
program. Each instruction waits for the previous instruction to complete its execution.

Asynchronous
Due to synchronous programming, sometimes important instruction gets blocked due to
some previous instructions, which causes a delay in the UI. Asynchronous code execution
allows you to execute the next instructions immediately and doesn’t block the flow.

See the output in console. If will display after two seconds.


We can write above code as:
This code does not wait for hello function but display rest result. It is called async
programming
Callbacks
A callback is a function passed as an argument to another function.

Function can’t be passed with ( ) parenthesis.


We can write above code as :

Callback Hell
Nested callbacks stacked below one another forming a pyramid structure. (Pyramid of
Doom). This style of programming becomes difficult to understand & manage.
Give some code examples of nested if, for etc.
For example we need in a sequence, first we need data of Id = 1, then id = 2, and id = 3;

But above code give data at the same time.


We can write above code as follows

This code will generate error


We resolve this can write if statement, to which whether the function is exit then call it.

We can do it for data id = 3, and 4 and so on….

this messy code is difficult to understand, so it is called callback hell.


Promises
Promise is for “eventual” completion of task. It is an object in JS.
It is a solution to callback hell.
See the return values on browser console. By writing -> promise
Promise has three states

See on console of browser, the promise will return ‘success’ message and also status –
fulfilled.

See on console of browser, the promise will return ‘rejected’ message and also status –
‘rejected’.

See the result on console, three states, pending, fulfill, and rejected
General programing is return automatically like API
How to Use Promises
these functions automatically have two parameters, which return res or err

Promise Chain
We write code one after another

We can place p2 in then block of p1


Return data one after another by following code

And refined promise chaining code


Async and Await
Async function always returns a promise
Async function myFunc(){…}
Awaits pauses the execution of its surrounding async function until the promise is settled.

It will return promise, check on console

Multiple call

Our first example


IIFE : Immediately Invoked Function Expression
IIFE is a function that is called immediately as soon as it is defined

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