JavaScript内部函数和“此”

Question

In my understanding, it is not until an object invokes a Function that this is actually assigned a value. And the value it is assigned is based exclusively on the object that invokes the Function.

Also, the scope chain rule in JS is LEG.

So, in (strict mode):

function one () {
    var a = 2;
    function two () {
        console.log(a)};
    two()
}

one() // output 2

But:

function one () {
    var a = 2;
    function two () {
        console.log(this.a)};
    two()
}

one()  //output undefined

It was my understandig functions were objects, and in the previous invokation the function object one would be the making the call of two , which translates this.a into one.a . Obviously that is not the case.

Also:

function one () {
        var a = 2}

console.log(one.a)  //outputs undefined

Any clarification about what is happening would be very appreciated.

Thanks

Answer 1

function one () {
    var a = 2;
    function two () {
        console.log(this.a)};
    two()
}

one()  //output undefined

Here you are calling both one and two as functions on their own, not as properties of some object (eg someObject.one() ). This means that this will refer to the global scope (or to undefined if the code is in strict mode). The a property of your global scope is undefined , so that's why you see undefined . Calling two() inside of one() doesn't make it so that this refers to one .

function one () {
        var a = 2}

console.log(one.a)  //outputs undefined

a is not a property of one . It is a variable inside it . A property of one would look like this.

 function one() { } one.a = 7; console.log(one.a); 

Answer 2

I think you are treating a regular function as a class object. You only call one() but you do not actually instantiate a new One() object.

Answer 3

Scope and Context are different concepts. Within a JS-Function scope one can address any value that either got assigned inside a function or any value that got assigned outside such a function as long as this function itself is a member of the outer scope. In addition, as long as a function does not create a closure, scope gets created just at a function's call time. Context is addressable too. But unlike scope , context gets evaluated at a functions call time. Thus this is just a placeholder for context that can vary from one time calling a function to another.

 function contextAwareMethod(injectedValue) { "use strict"; var context = this, // dynamically evaluated. innerScopeValue = "created at a functions call time"; console.log('dynamically evaluated context : ', context); console.log('this.contextValue : ', (context && context.value)); console.log('innerScopeValue : ', innerScopeValue); console.log('injectedValue : ', injectedValue); } console.log('"case A"'); contextAwareMethod('argument A'); console.log('\\n'); console.log('"case B"'); contextAwareMethod.call({ value: "conext B"}); console.log('\\n'); console.log('"case C"'); contextAwareMethod.call({ value: "conext C"}, 'argument C'); console.log('\\n'); 

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