“ this”是指在私有函数中调用时的窗口

Question

I'm still wrapping my head around "this". Here I use "this" inside of a function sayFoo .

I'm surprised that "this" is the window, when I log it from inside sayFoo().

What am I not understanding here about context?

myCoolThing = {
  foo: "1",
  doSomething: function() {
    sayFoo();

    function sayFoo() {
      console.log(this.foo);
    }
  }
}

myCoolThing.doSomething(); // Errors with "foo" undefined

Answer 1

When you call sayFoo , it is called without the object as the context. Only functions which are properties of the object are called with the context as the parent object. This would work:

myCoolThing = {
  foo: "1",
  doSomething: function() {
    sayFoo.call(this);

    function sayFoo() {
      console.log(this.foo);
    }
  }
}

More info on context .

Function.prototype.call() simply calls the function with this set to the first parameter it is called with (which, in this case, is the object since that is what this is in a function which is a property of an object).