Understand JavaScript’s “this” With Clarity, and Master It

The this keyword in JavaScript confuses new and seasoned JavaScript developers alike. This article aims to elucidate this in its entirety.

By the time we make it through this article, this will be one part of JavaScript we never have to worry about again. We will understand how to use thiscorrectly in every scenario, including the ticklish situations where it usually proves most elusive.

We use this similar to the way we use pronouns in natural languages like English and French. We write, “John is running fast because he is trying to catch the train.” Note the use of the pronoun “he.” We could have written this: “John is running fast because John is trying to catch the train.” We don’t reuse “John” in this manner, for if we do, our family, friends, and colleagues would abandon us. Yes, they would. Well, maybe not your family, but those of us with fair-weather friends and colleagues.

In a similar graceful manner, in JavaScript, we use the this keyword as a shortcut, a referent; it refers to an object; that is, the subject in context, or the subject of the executing code. Consider this example:

      var person = {
      firstName: "Penelope",
      lastName: "Barrymore",
      fullName: function () {
      ​// Notice we use "this" just as we used "he" in the example sentence earlier?:​
      console.log(this.firstName + " " + this.lastName);
      ​// We could have also written this:​​
      console.log(person.firstName + " " + person.lastName);
      }
      }

If we use person.firstName and person.lastName, as in the last example, our code becomes ambiguous. Consider that there could be another global variable (that we might or might not be aware of) with the name “person.” Then, references to person.firstName could attempt to access the fistName property from the person global variable, and this could lead to difficult-to-debug errors. So we use the “this” keyword not only for aesthetics (i.e., as a referent), but also for precision; its use actually makes our code more unambiguous, just as the pronoun “he” made our sentence more clear. It tells us that we are referring to the specific John at the beginning of the sentence.(使用this不仅美观而且精确，因为如果用person.firstName来代替this.firstName，如果还存在一个person全局变量，那么将会报解析错误。)

Just like the pronoun “he” is used to refer to the antecedent (antecedent is the noun that a pronoun refers to), the this keyword is similarly used to refer to an object that the function (wherethis is used) is bound to. The this keyword not only refers to the object but it also contains the value of the object. Just like the pronoun, this can be thought of as a shortcut (or a reasonably unambiguous substitute) to refer back to the object in context (the “antecedent object”). We will learn more about context later.

JavaScript’s this Keyword Basics

First, know that all functions in JavaScript have properties, just as objects have properties.(因为Function也是对象) And when a function executes, it gets the this property—a variable with the value of the object that invokes the function where this is used.(当函数执行的时候，函数会获得一个this属性，this是一个保存了object的变量，当this使用的时候，将会执行函数)

The this reference ALWAYS refers to (and holds the value of) an object—a singular object—and it is usually used inside a function or a method, although it can be used outside a function in the global scope. Note that when we use strict mode, this holds the value of undefined in global functions and in anonymous functions that are not bound to any object.

this is used inside a function (let’s say function A) and it contains the value of the object that invokes function A. We need this to access methods and properties of the object that invokes function A, especially since we don’t always know the name of the invoking object, and sometimes there is no name to use to refer to the invoking object. Indeed, this is really just a shortcut reference for the “antecedent object”—the invoking object.

Ruminate on this basic example illustrating the use of this in JavaScript:

     var person = {
     firstName :"Penelope",
     lastName :"Barrymore",
     // Since the "this" keyword is used inside the showFullName method below, and the showFullName method is defined on the person object,​
     // "this" will have the value of the person object because the person object will invoke showFullName ()​
     showFullName:function () {
     console.log (this.firstName + " " + this.lastName);
     }
     ​
     }
     ​
     person.showFullName (); // Penelope Barrymore

And consider this basic jQuery example with of this:

     // A very common piece of jQuery code​
     ​
     $ ("button").click (function (event) {
     // $(this) will have the value of the button ($("button")) object​
     ​// because the button object invokes the click () method​
     console.log ($ (this).prop ("name"));
     });

I shall expound on the preceding jQuery example: The use of $(this), which is jQuery’s syntax for the this keyword in JavaScript, is used inside an anonymous function, and the anonymous function is executed in the button’s click () method. The reason $(this) is bound to the button object is because the jQuery library binds $(this) to the object that invokes the click method. Therefore, $(this) will have the value of the jQuery button ($(“button”)) object, even though $(this) is defined inside an anonymous function that cannot itself access the “this” variable on the outer function.

Note that the button is a DOM element on the HTML page, and it is also an object; in this case it is a jQuery object because we wrapped it in the jQuery $() function.

The Biggest Gotcha with JavaScript “this” keyword

If you understand this one principle of JavaScript’s this, you will understand the “this” keyword with clarity: this is not assigned a value until an object invokes the function where this is defined.(只有当一个对象调用一个函数，且函数中定义了this时，this才会被赋值) Let’s call the function where this is defined the “this Function.”

Even though it appears this refers to the object where it is defined, it is not until an object invokes the this Function that this is actually assigned a value. And the value it is assigned is based exclusively on the object that invokes the this Function. this has the value of the invoking object in most circumstances. However, there are a few scenarios where this does not have the value of the invoking object. I touch on those scenarios later.

The use of this in the global scope

In the global scope, when the code is executing in the browser, all global variables and functions are defined on the window object. Therefore, when we use this in a global function, it refers to (and has the value of) the global window object (not in strict mode though, as noted earlier) that is the main container of the entire JavaScript application or web page.

Thus:

     var firstName = "Peter",
     lastName = "Ally";
     ​
     function showFullName () {
     // "this" inside this function will have the value of the window object​
     // because the showFullName () function is defined in the global scope, just like the firstName and lastName​
     console.log (this.firstName + " " + this.lastName);
     }
     ​
     var person = {
     firstName :"Penelope",
     lastName :"Barrymore",
     showFullName:function () {
     // "this" on the line below refers to the person object, because the showFullName function will be invoked by person object.​
     console.log (this.firstName + " " + this.lastName);
     }
     }
     ​
     showFullName (); // Peter Ally​
     ​
     // window is the object that all global variables and functions are defined on, hence:​
     window.showFullName (); // Peter Ally​
     ​
     // "this" inside the showFullName () method that is defined inside the person object still refers to the person object, hence:​
     person.showFullName (); // Penelope Barrymore

When this is most misunderstood and becomes tricky

The this keyword is most misunderstood when we borrow a method that uses this, when we assign a method that uses this to a variable, when a function that uses this is passed as a callback function, and when this is used inside a closure—an inner function. We will look at each scenario and the solutions for maintaining the proper value of this in each example.

A bit about “Context” before we continue
The context in JavaScript is similar to the subject of a sentence in English: “John is the winner who returned the money.” The subject of the sentence is John, and we can say thecontext of the sentence is John because the focus of the sentence is on him at this particular time in the sentence. Even the “who” pronoun is referring to John, the antecedent. And just like we can use a semicolon to switch the subject of the sentence, we can have an object that is current context and switch the context to another object by invoking the function with another object.

Similarly, in JavaScript code:

    var person = {
     firstName :"Penelope",
     lastName :"Barrymore",
     showFullName:function () {
     ​// The "context"​
     console.log (this.firstName + " " + this.lastName);
     }
     }
     ​
     ​// The "context", when invoking showFullName, is the person object, when we invoke the showFullName () method on the person object.​
     ​// And the use of "this" inside the showFullName() method has the value of the person object,​
     person.showFullName (); // Penelope Barrymore​
     ​
     ​// If we invoke showFullName with a different object:​
     ​var anotherPerson = {
     firstName :"Rohit",
     lastName :"Khan"​
     };
     ​
     ​// We can use the apply method to set the "this" value explicitly—more on the apply () method later.​
     ​// "this" gets the value of whichever object invokes the "this" Function, hence:​
     person.showFullName.apply (anotherPerson); // Rohit Khan​
     ​
     ​// So the context is now anotherPerson because anotherPerson invoked the person.showFullName () method by virtue of using the apply () method​

The takeaway is that the object that invokes the this Function is in context, and we can change the context by invoking the this Function with another object; then this new object is in context.

Here are scenarios when the this keyword becomes tricky. The examples include solutions to fix errors with this:

1. Fix this when used in a method passed as a callback

   Things get a touch hairy when we pass a method (that uses this) as a parameter to be used as a callback function. For example:

        // We have a simple object with a clickHandler method that we want to use when a button on the page is clicked​
        var user = {
        data:[
        {name:"T. Woods", age:37},
        {name:"P. Mickelson", age:43}
        ],
        clickHandler:function (event) {
        var randomNum = ((Math.random () * 2 | 0) + 1) - 1; // random number between 0 and 1​
        ​
        // This line is printing a random person's name and age from the data array​
        console.log (this.data[randomNum].name + " " + this.data[randomNum].age);
        }
        }
        ​
        // The button is wrapped inside a jQuery $ wrapper, so it is now a jQuery object​
        // And the output will be undefined because there is no data property on the button object​
        $ ("button").click (user.clickHandler); // Cannot read property '0' of undefined

   In the code above, since the button ($(“button”)) is an object on its own, and we are passing the user.clickHandler method to its click() method as a callback, we know that this inside ouruser.clickHandler method will no longer refer to the user object. this will now refer to the object where the user.clickHandler method is executed because this is defined inside the user.clickHandler method. And the object that is invoking user.clickHandler is the button object—user.clickHandler will be executed inside the button object’s click method.

   Note that even though we are calling the clickHandler () method with user.clickHandler (which we have to do, since clickHandler is a method defined on user), the clickHandler () method itself will be executed with the button object as the context to which “this” now refers. So this now refers to is the button ($(“button”)) object.

   At this point, it should be apparent that when the context changes—when we execute a method on some other object than where the object was originally defined, the this keyword no longer refers to the original object where “this” was originally defined, but it now refers to the object that invokes the method where this was defined.

   Solution to fix this when a method is passed as a callback function:
   Since we really want this.data to refer to the data property on the user object, we can use the Bind (), Apply (), or Call () method to specifically set the value of this.

   I have written an exhaustive article, JavaScript’s Apply, Call, and Bind Methods are Essential for JavaScript Professionals, on these methods, including how to use them to set the thisvalue in various misunderstood scenarios. Rather than re-post all the details here, I recommend you read that entire article, which I consider a must read for JavaScript Professionals.

   To fix this problem in the preceding example, we can use the bind method thus:

   Instead of this line:

        $ ("button").click (user.clickHandler);

   We have to bind the clickHandler method to the user object like this:

        $("button").click (user.clickHandler.bind (user)); // P. Mickelson 43

2. Fix this inside closure

   Another instance when this is misunderstood is when we use an inner method (a closure). It is important to take note that closures cannot access the outer function’s this variable by using the this keyword because the this variable is accessible only by the function itself, not by inner functions. For example:

        var user = {
        tournament:"The Masters",
        data :[
        {name:"T. Woods", age:37},
        {name:"P. Mickelson", age:43}
        ],
        ​
        clickHandler:function () {
        // the use of this.data here is fine, because "this" refers to the user object, and data is a property on the user object.​
        ​
        this.data.forEach (function (person) {
        // But here inside the anonymous function (that we pass to the forEach method), "this" no longer refers to the user object.​
        // This inner function cannot access the outer function's "this"​
   
        console.log ("What is This referring to? " + this); //[object Window]​
   
        console.log (person.name + " is playing at " + this.tournament);
        // T. Woods is playing at undefined​
        // P. Mickelson is playing at undefined​
        })
        }
        ​
        }
        ​
        user.clickHandler(); // What is "this" referring to? [object Window]

   this inside the anonymous function cannot access the outer function’s this, so it is bound to the global window object, when strict mode is not being used.

   Solution to maintain this inside anonymous functions:
   To fix the problem with using this inside the anonymous function passed to the forEachmethod, we use a common practice in JavaScript and set the this value to another variable before we enter the forEach
   method:

        var user = {
        tournament:"The Masters",
        data :[
        {name:"T. Woods", age:37},
        {name:"P. Mickelson", age:43}
        ],
        ​
        clickHandler:function (event) {
        // To capture the value of "this" when it refers to the user object, we have to set it to another variable here:​
        // We set the value of "this" to theUserObj variable, so we can use it later​
        var theUserObj = this;
        this.data.forEach (function (person) {
        // Instead of using this.tournament, we now use theUserObj.tournament​
        console.log (person.name + " is playing at " + theUserObj.tournament);
        })
        }
        ​
        }
        ​
        user.clickHandler();
        // T. Woods is playing at The Masters​
        // P. Mickelson is playing at The Masters

   It is worth noting that many JavaScript developers like to name a variable “that,” as seen below, to set the value of this. The use of the word “that” is very awkward for me, so I try to name the variable a noun that describes which object “this” is referring to, hence my use ofvar theUserObj = this in the preceding code.

        // A common practice amongst JavaScript users is to use this code​
        var that = this;

3. Fix this when method is assigned to a variable

   The this value escapes our imagination and is bound to another object, if we assign a method that uses this to a variable. Let’s see how:

        // This data variable is a global variable​
        var data = [
        {name:"Samantha", age:12},
        {name:"Alexis", age:14}
        ];
        ​
        var user = {
        // this data variable is a property on the user object​
        data :[
        {name:"T. Woods", age:37},
        {name:"P. Mickelson", age:43}
        ],
        showData:function (event) {
        var randomNum = ((Math.random () * 2 | 0) + 1) - 1; // random number between 0 and 1​
        ​
        // This line is adding a random person from the data array to the text field​
        console.log (this.data[randomNum].name + " " + this.data[randomNum].age);
        }
        ​
        }
        ​
        // Assign the user.showData to a variable​
        var showUserData = user.showData;
        ​
        // When we execute the showUserData function, the values printed to the console are from the global data array, not from the data array in the user object​
        //​
        showUserData (); // Samantha 12 (from the global data array)​
        ​

   Solution for maintaining this when method is assigned to a variable:
   We can fix this problem by specifically setting the this value with the bind method:

        // Bind the showData method to the user object​
        var showUserData = user.showData.bind (user);
        ​
        // Now we get the value from the user object, because the <em>this</em> keyword is bound to the user object​
        showUserData (); // P. Mickelson 43

4. Fix this when borrowing methods

   Borrowing methods is a common practice in JavaScript development, and as JavaScript developers, we will certainly encounter this practice time and again. And from time to time, we will engage in this time-saving practice as well. For more on borrowing methods, read my in-depth article, JavaScript’s Apply, Call, and Bind Methods are Essential for JavaScript Professionals.

   Let’s examine the relevance of this in the context of borrowing methods:

        // We have two objects. One of them has a method called avg () that the other doesn't have​
        // So we will borrow the (avg()) method​
        var gameController = {
        scores :[20, 34, 55, 46, 77],
        avgScore:null,
        players :[
        {name:"Tommy", playerID:987, age:23},
        {name:"Pau", playerID:87, age:33}
        ]
        }
        ​
        var appController = {
        scores :[900, 845, 809, 950],
        avgScore:null,
        avg :function () {
        ​
        var sumOfScores = this.scores.reduce (function (prev, cur, index, array) {
        return prev + cur;
        });
        ​
        this.avgScore = sumOfScores / this.scores.length;
        }
        }
        ​
        //If we run the code below,​
        // the gameController.avgScore property will be set to the average score from the appController object "scores" array​
   
        // Don't run this code, for it is just for illustration; we want the appController.avgScore to remain null​
        gameController.avgScore = appController.avg();
        ​

   The avg method’s “this” keyword will not refer to the gameController object, it will refer to the appController object because it is being invoked on the appController.

   Solution for fixing this when borrowing methods:
   To fix the issue and make sure that this inside the appController.avg () method refers to gameController, we can use the apply () method thus:

        ​
        // Note that we are using the apply () method, so the 2nd argument has to be an array—the arguments to pass to the appController.avg () method.​
        appController.avg.apply (gameController, gameController.scores);
        ​
        // The avgScore property was successfully set on the gameController object, even though we borrowed the avg () method from the appController object​
        console.log (gameController.avgScore); // 46.4​
        ​
        // appController.avgScore is still null; it was not updated, only gameController.avgScore was updated​
        console.log (appController.avgScore); // null

   The gameController object borrows the appController’s avg () method. The “this” value inside the appController.avg () method will be set to the gameController object because we pass the gameController object as the first parameter to the apply () method. The first parameter in the apply method always sets the value of “this” explicitly.

Final Words

I am hopeful you have learned enough to help you understand the this keyword in JavaScript. Now you have the tools (bind, apply, and call, and setting this to a variable) necessary to conquer JavaScript’s this in every scenario.

As you have learned, this gets a bit troublesome in situations where the original context (wherethis was defined) changes, particularly in callback functions, when invoked with a different object, or when borrowing methods. Always remember that this is assigned the value of the object that invoked the this Function.

Be good. Sleep well. And enjoy coding.

原文链接：http://javascriptissexy.com/understand-javascripts-this-with-clarity-and-master-it/