The invocation context (this) of the forEach funct

Question

I was wondering what the 'this' value (or invocation context) is of the forEach callback function. This code doesn't seem to work:

var jow = [5, 10, 45, 67];

jow.forEach(function(v, i, a){

    this[i] = v + 1;

});

alert(jow);

Thx for explaining it to me.

Answer 1

If you dont pass second parameter to forEach, this will point to the global object. To achieve what you were trying to do

var jow = [5, 10, 45, 67];

jow.forEach(function(v, i, a) {
    a[i] = v + 1;
});

console.log(jow);

Output

[ 6, 11, 46, 68 ]

Answer 2

MDN states:

array.forEach(callback[, thisArg])

If a thisArg parameter is provided to forEach, it will be used as the this value for each callback invocation as if callback.call(thisArg, element, index, array) was called. If thisArg is undefined or null, the this value within the function depends on whether the function is in strict mode or not (passed value if in strict mode, global object if in non-strict mode).

So in short, if you only provide the callback and you're in non-strict mode (the case you presented), it will be the global object (window).

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

Answer 3

I finished construction of the forEach method and wanted to share this diagram with everyone, hope it helps someone else trying to understand its inner workings.

Answer 4

Inside forEach, this refers to the global window object. This is the case even if you call it from a different object (i.e. one you've created)

window.foo = 'window';

var MyObj = function(){
  this.foo = 'object';
};

MyObj.prototype.itirate = function () {
  var _this = this;

  [''].forEach(function(val, index, arr){
    console.log('this: ' + this.foo); // logs 'window'
    console.log('_this: ' + _this.foo); // logs 'object'
  });
};

var newObj = new MyObj();

newObj.itirate();
// this: window
// _this: object