Objectives:

By the end of this chapter, you should be able to:

• Explain how the keyword this is determined with implicit binding
• Discuss this difference between the default and implicit binding of the keyword this

We've seen how the default binding works. But how can the keyword this change its value based on the surrounding context?

Implicit Binding

One option is that the function wrapping the keyword this is being called as a method on some object. In this case the call-site has some parent object, and according to the implicit binding rule, this should refer to this parent object. Anytime you see the keyword this, you should check to see whether the closing function has an associated parent object when the function is called. In other words, check the call-site!

Here are a few examples:

var friend = {
    firstName: "Ash",
    sayHi: function(){
        return this.firstName + " says hello!";
    }
};

// the keyword this will refer to friend when we invoke sayHi:
friend.sayHi(); // Ash says hello!

var dog = {
    name: "Whiskey",
    sleep: function() {
        return this.name + " is sleeping: zzzzz...."
    }
}

dog.sleep(); // Whiskey is sleeping: zzzzz....

var nested = {
    number: 1,
    anotherObject: {
        anotherNumber: 2,
        whatIsThis: function() {
            return this;
        }
    }
}

nested.anotherObject.whatIsThis(); // Object {anotherNumber: 2}