this value in JavaScript anonymous function

Question

Can anybody explain to me why A is true and B is false? I would have expected B to be true as well.

function MyObject() {

};

MyObject.prototype.test = function () {
    console.log("A", this instanceof MyObject);
    (function () {
        console.log("B", this instanceof MyObject);
    }());
}

new MyObject().test();

update: since ecmascript-6 you can use arrow functions which would make it easy to refer to MyObject like this:

function MyObject() {

};

MyObject.prototype.test = function () {
    console.log("A", this instanceof MyObject);
    (() => {//a change is here, which will have the effect of the next line resulting in true
        console.log("B", this instanceof MyObject);
    })(); //and here is a change
}

new MyObject().test();    

Answer 1

Inside of your anonymous function this is the global object.

Inside of test, this is the instance of MyObject on which the method was invoked.

---

Whenever you call a function like this:

somceFunction(); // called function invocation

this is always the global object, or undefined in strict mode (unless someFunction was created with bind^** — see below)

Whenever you call a function like this

foo.someMethod();  //called method invocation

this is set to foo

---

^**EcmaScript5 defines a bind function that allows you to create a function that has a pre-set value for this

So this

    var obj = { a: 12 };
    var someFunction = (function () { alert(this.a); }).bind(obj);
    someFunction();

Causes someFucntion to be invoked with this equal to obj, and alerts 12. I bring this up only to note that this is a potential exception to the rule I mentioned about functions invoked as

someFunction();

always having this equal to the global object (or undefined in strict mode)