javascript - js call execution process

Question

window._ = {
    VARSION:"0.1.0",
    each:function(obj,iterator,context){
        var index = 0;
        try{
            if(obj.forEach){
                obj.forEach(iterator,context);
            }else if(obj.length){
                for( var i= 0; i<obj.length; i++){
                    iterator.call(context,obj[i],i);
                }
            }else if(obj.each){
                obj.each(function(value){
                    iterator.call(context,value,index++)
                });
            }else{
                var i = 0;
                for(var key in obj){
                    var value = obj[key],
                        pair = [key,value];
                    pair.key = key;
                    pair.value = value;
                    iterator.call(context,pair,i++);
                }
            }
        }catch(e){
            console.log(e)
            // if(e != "__break__") throw e;
        }
        return obj;
    }
}
var arr = {
    a:5,
    b:6,
    c:4
}
_.each(arr,function(a,b){
    console.log(a)
    console.log(b)
})

Img cannot be uploaded due to network speed. Sorry
I want to know the specific function of that call in this code.
What is the execution process? Thank you all

Answer 1

The scope of

call 是为了给你保证你提供了第三个参数的时候 callback is not polluted.

_.each(arr, function (a, b) {
    console.log(this); //window
})

_.each(arr, function (a, b) {
    console.log(this); // arr
},arr)

The execution order of the code can be debug

Answer 2

iterator.call(context...
is equivalent to binding this to the iterator function as context

Answer 3

iterator.call() 中，iterator is the traversal function passed in. In this case, it refers to the anonymous function:

function(a, b) { console.log(a); console.log(b); }

Therefore, call refers to Function.prototype.call . For details, see Function.prototype.call() - JavaScript | MDN

The signature format of

Function.prototype.call is:

func.call(thisArg, param1, param2, ...)

thisArg is used to change the binding of the this pointer inside the function.

Answer 4

Used to specify the execution environment of the function

Answer 5

call makes this of the specified function point to the corresponding object.
The above example:
iterator.call(context,obj[i],i)//this points to context, obj[i],i is the parameter

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