The invocation context (this) of the forEach function call

javascript function foreach this invocation
kevinius picture kevinius · Oct 31, 2013 · Viewed 35.7k times · Source

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

Tibos picture Tibos · Oct 31, 2013

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

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