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_Objects/Array/forEach, But it doesn't work. It works if i use call with it.
As i understand call and apply are used to supply this and arguments to function. However, there should be no need. So what am i missing?
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}
It's already in the documentation (emphasis mine):
15.4.4.18
Array.prototype.forEach ( callbackfn [ , thisArg ] )
callbackfnshould be a function that accepts three arguments.forEachcallscallbackfnonce for each element present in the array, in ascending order.callbackfnis called only for elements of the array which actually exist; it is not called for missing elements of the array.If a
thisArgparameter is provided, it will be used as thethisvalue for each invocation ofcallbackfn. If it is not provided,undefinedis 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.
forEach version (which shows the problem immediately)function forEach( 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]"
>>>. Seeing it first time this year. Lots of googling and still in dark. What's the purpose of it here?...
>>> is zero-fill right shift (JS operators reference). In this case it's used as a slightly more compact (but substantially more confusing) way to say Number(this.length), casting this.length to a number in case it happens to be a string.
Number can result in a double precision value. That's a verbatim translation of the original step "Let len be ToUint32(lenValue)."