JavaScript Object Methods

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The this Object

Every scope in JavaScript has a this object that represents the calling object for the function. In the global scope, this represents the global object (window in web browsers). When a function is called while attached to an object, the value of this is equal to that object by default. So, instead of directly referencing an object inside a method, you can reference this instead. For example:

var person = {
    name: "Michael",
    sayName: function () {
        console.log(this.name);
    }
};

person.sayName();       // outputs "Michael"

You can easily change the name of the variable or even reuse the function on different objects.

function sayNameForAll() {
    console.log(this.name);
}

var person1 = {
    name: "Michael",
    sayName: sayNameForAll
};

var person2 = {
    name: "Kobe",
    sayName: sayNameForAll
};

var name = "David";

person1.sayName();      // outputs "Micheal"
person2.sayName();      // outputs "Kobe"

sayNameForAll();        // outputs "David"

The ability to use and manipulate the this value of functions is key to good object-oriented programming in JavaScript. Functions can be used in many different contexts, and they need to be able to work in each situation. Even though this is typically assigned automatically, you can change its value to achieve different goals. There are three function methods that allow you to change the value of this. (Remember that functions are objects, and objects can have methods, so functions can, too.)

The call() Method

The first function method for manipulating this is call(), which executes the function with a particular this value and with specific parameters. The first parameter of call() is the value to which this should be equal when the function is executed. All subsequent parameters are the parameters that should be passed into the function. For example:

function sayNameForAll(label) {
    console.log(label + ":" + this.name);
}

var person1 = {
    name: "Michael"
};

var person2 = {
    name: "Kobe"
};

var name = "David";

sayNameForAll.call(this, "global");         // outputs "global:Michael"
sayNameForAll.call(person1, "person1");     // outputs "person1:Kobe"
sayNameForAll.call(person2, "person2");     // outputs "person2:David"

The apply() Method

The second function method you can use to manipulate this is apply(). The apply() method works exactly the same as call() except that it accepts only two parameters: the value for this and an array or array-like object of parameters to pass to the function (that means you can use an arguments object as the second parameter). So, instead of individually naming each parameter using call(), you can easily pass arrays to apply() as the second argument. Otherwise, call() and apply() behave identically. For example: 

function sayNameForAll(label) {
    console.log(label + ":" + this.name);
}

var person1 = {
    name: "Michael"
};

var person2 = {
    name: "Kobe"
};

var name = "David";

sayNameForAll.apply(this, ["global"]);         // outputs "global:Michael"
sayNameForAll.apply(person1, ["person1"]);     // outputs "person1:Kobe"
sayNameForAll.apply(person2, ["person2"]);     // outputs "person2:David"

Note:

The method you use typically depends on the type of data you have. If you already have an array of data, use apply(); if you just have individual variables, use call().

The bind() Method

The third function method for changing this is bind(). This method was added in ECMAScript 5, and it behaves quite differently than the other two. The first argument to bind() is the this value for the new function. All other arguments represent named parameters that should be permanently set in the new function. You can still pass in any parameters that aren't permanently set later.

The following code shows two examples that use bind(). You create the sayNameForPerson1() function by binding the this value to person1, while sayNameForPerson2() binds this to person2 and binds the first parameter as "person2".

function sayNameForAll(label) {
    console.log(label + ":" + this.name);
}

var person1 = {
    name: "Michael"
};

var person2 = {
    name: "Kobe"
};

// create a function just for person1
var sayNameForPerson1 = sayNameForAll.bind(person1);
sayNameForPerson1("person1");       // outputs "person1:Michael"

// create a function just for person2
var sayNameForPerson2 = sayNameForAll.bind(person2, "person2");
sayNameForPerson2();                // outputs "person2:Kobe"

// attaching a method to an object doesn't change 'this'
person2.sayName = sayNameForPerson1;
person2.sayName("person2");         // outputs "person2:Michael"

No parameters are bound for sayNameForPerson1(), so you still need to pass in the label for the output. The function sayNameForPerson2() not only binds this to person2 but also binds the first parameter as "person2". That means you can call sayNameForPerson2() without passing in any additional arguments. The last part of this example adds sayNameForPerson1() onto person2 with the name sayName. The function is bound, so the value of this doesn’t change even though sayNameForPerson1 is now a function on person2. The method still outputs the value of person1.name.

Reference

Leanpub.Principles.of.Object-Oriented.Programming.in.JavaScript.Jun.2014