What Is A Difference Between Array.prototype.foreach.call Vs Array.prototype.foreach

According to documentation at MDN you can provide this argument to forEach function if invoked like this, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_O

Solution 1:

It definitely does work, the second parameter to forEach supplies context to the callback

var numbers = [1,2,3,4];
var sum = {value: 0};

numbers.forEach(function(num){ 
   this.value += num;
}, sum);

console.log(sum); // Object {value: 10}

Solution 2:

Explanation

It's already in the documentation (emphasis mine):

15.4.4.18 Array.prototype.forEach ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

thisArg is only used in the invocation of callbackfn. It's not, however, used to provide the this value for forEach, where this needs to be an array like structure (that means it has a length property). If you use Array.prototype.forEach(..., someObject), the this value in forEach's context will be undefined.

Simplified forEach version (which shows the problem immediately)

functionforEach( callback , thisArg ){
    // The algorithm parses the length as UInt32, see step 3.// >>> not only acts as bit shift, but also forces the // value into an unsigned number.var len = this.length >>> 0, // using "this", not "thisArg"!
        i;

    for(i = 0; i < len; ++i){
        callback.call(thisArg, this[i]);
        // thisArg   ^^^^^^^^^ is used here, not up at length
    }
}

// example calls:var logArguments = function(args){
    console.log(args, this);
}
forEach(logArguments, [1,2,3]); // logs nothing
forEach.call([1,2,3], logArguments); // logs 1, 2, 3
forEach.call([1,2,3], logArguments, [2,3,4]); // logs "1 Array [2,3,4]"//      "2 Array [2,3,4]"//      "3 Array [2,3,4]"