module.exports = index;

/*

index([{id: 'first', val: 1}, {id: 'second', val: 2}], 'id');

// {first: {id: 'first', val: 1}, second: {id: 'second', val: 2}}

index([{id: 'first', val: 1}, null], 'id'); // {first: {id: 'first', val: 1}}

index([], 'id'); // {}

index([], null); // throws

index({}, 'id'); // throws

*/

function index(arr, key) {

if (!Array.isArray(arr)) {

throw new Error('expected an array for first argument');

}

if (typeof key != 'string') {

throw new Error('expected a string for second argument');

}

var result = {};

var len = arr.length;

for (var i = 0; i < len; i++) {

var index = arr[i] && arr[i][key];

if (index) {

result[index] = arr[i];

}

}

return result;

}