Does forEach create a deep copy of array before looping?

Question

Here is an example

arr1 = [{ b: 2 }, { a: 1 }] // an array with 2 elements

arr1.forEach(function (element, index, array) {

    console.log(element);
    console.log('of');
    console.log(array);
    console.log('');


    arr1.push({ c: 3 });
});

console.log(arr1);

Result

{ b: 2 }
of
[ { b: 2 }, { a: 1 } ]

{ a: 1 }
of
[ { b: 2 }, { a: 1 }, { c: 3 } ]

[ { b: 2 }, { a: 1 }, { c: 3 }, { c: 3 } ]

In above example I am traversing an array and adding more values to it and they are getting added in the original one while looping

Does forEachuse a different array to loop?

Answer 1

It does not use a different array, as you can see, that when you console.log(array);, you will still see the new elements, even though you pushed them onto arr1. So we know that array and arr1 point to the same Array.

However what forEach does, atleast according to the polyfill on MDN, is this:

Before iterating through it, it will extract the length of the Array, and only then start iterating. So if the length of the array changes inside the function you pass to forEach, the iteration will not change.

// 2. Let lenValue be the result of calling the Get() internal
// method of O with the argument "length".
// 3. Let len be toUint32(lenValue).
var len = O.length >>> 0;



// 6. Let k be 0
k = 0;

// 7. Repeat, while k < len
while (k < len) {
    ...
}