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obj.getFoo() returns an equation assigned to f .
f is called via f() without an explicit caller, so this is just window .

If you want to get "obj", you can do this

var foo = "window";
var obj = {
    foo : "obj",
    getFoo : function(){
        var self = this;
        return function(){
            return self.foo;
        };
    }
};

var f = obj.getFoo();
console.log(f());

Because, where f() actually runs, this is window. Since the context is not changed through call or bind, the output is window.

You can replace it as follows:

console.log(f());
// ----->
console.log(obj.getFoo());
// ----->
console.log(function() {
    var self = this;
    return function() {
        return self.foo
    }
});

The self here points to window, so return self.foo is return window.foo, which is 'window'.

In fact, the simplest understanding is that obj.getFoo gives f, and then look at where this method runs.

f = function () {
    return function () {
        return this.foo
    }
}

f() in console.log(f()) is called independently
1. If the caller function is owned by an object, then when the function is called, the internal this points to the object.
2. If the function is called independently, then this inside the function points to undefined.
Recommended reading http://www.jianshu.com/p/d647... I hope it will be helpful to you

Function execution, this in the function body points to the caller of the function

1. In the following code, the caller of the getFoo function is obj, so this inside the getFoo function points to the obj object

var f = obj.getFoo()

2. The getFoo function returns an anonymous function and assigns it to the variable f, and then executes the function f. At this time, the variable f is mounted on the window. The caller of the function f is the window, and this inside the function f also points to the window

console.log(f());  //結(jié)果：window