How to correctly handle this pointing in a closure?

The methods to correctly handle this pointing in JavaScript closures include: 1. Use arrow functions, 2. Use bind methods, 3. Use variables to save this. These methods ensure that this intrinsic function correctly points to the context of the external function.

introduction

Today we will discuss a problem that is often troublesome in JavaScript development: how to correctly handle this pointing in closures. I know that many developers are often confused when facing this problem, but don't worry, I will take you to solve this mystery step by step. Through this article, you will learn how to flexibly control this pointing in closures and master some practical tips and best practices.

Review of basic knowledge

In JavaScript, this is a very special keyword, and its pointing will vary according to different contexts. When we talk about closures, this pointing problem becomes particularly complicated. A closure is a function that has permission to access variables in another function scope, usually implemented by defining another function inside the function.

Before discussing this pointing, let's review the basic behavior of this :

• In a global environment, this points to a global object ( window in the browser and global in Node.js).
• When calling a function, the direction of this depends on the method of calling the function, such as direct call, call through object method, call using call or apply method, etc.

After understanding these basics, we can explore more in-depth how to properly handle this in closures.

Core concept or function analysis

this pointing problem in closure

In closures, the problem that this points to is mainly because this of the inner function is not synchronized with this of the outer function. Let's look at a simple example:

function outerFunction() {
    this.name = 'outer';
    function innerFunction() {
        console.log(this.name); // What does this point to here?
    }
    innerFunction();
}
<p>const obj = {
name: 'object'
};</p><p> outerFunction.call(obj); // Output: undefined</p>

In this example, this in innerFunction points to a global object, not this of outerFunction . This is because in non-strict mode, this in the internal function points to the global object by default.

Solution

To properly handle this pointer in a closure, we can use the following methods:

Use arrow functions

An important feature of arrow functions is that they do not have this of their own, but inherit this of the outer scope. This makes the arrow function very useful in closures:

function outerFunction() {
    this.name = 'outer';
    const innerFunction = () => {
        console.log(this.name); // This here points to this of outerFunction
    };
    innerFunction();
}
<p>const obj = {
name: 'object'
};</p><p> outerFunction.call(obj); // Output: outer</p>

Use bind method

The bind method allows us to create a new function whose this function is bound to the specified value:

function outerFunction() {
    this.name = 'outer';
    function innerFunction() {
        console.log(this.name);
    }
    innerFunction.bind(this)();
}
<p>const obj = {
name: 'object'
};</p><p> outerFunction.call(obj); // Output: outer</p>

Save this with variables

Another common method is to save this from an external function into a variable and then use this variable in the internal function:

function outerFunction() {
    this.name = 'outer';
    const self = this;
    function innerFunction() {
        console.log(self.name);
    }
    innerFunction();
}
<p>const obj = {
name: 'object'
};</p><p> outerFunction.call(obj); // Output: outer</p>

Example of usage

Basic usage

Let's look at an example of a practical application, suppose we want to create a counter class, where a method is used in the closure:

class Counter {
    constructor() {
        this.count = 0;
    }
<pre class='brush:php;toolbar:false;'>increment() {
    setTimeout(() => {
        this.count ;
        console.log(this.count);
    }, 1000);
}

}

const counter = new Counter(); counter.increment(); // Output after 1 second: 1

In this example, we use the arrow function to make sure this points to Counter instance.

Advanced Usage

In more complex scenarios, we may need to dynamically change the direction of this in the closure. For example, suppose we have a button click event handler and we want to update the status of an object when clicked:

class ButtonHandler {
    constructor(button) {
        this.button = button;
        this.clicks = 0;
        this.button.addEventListener('click', this.handleClick.bind(this));
    }
<pre class='brush:php;toolbar:false;'>handleClick() {
    this.clicks ;
    console.log(`Button clicked ${this.clicks} times`);
}

}

const button = document.getElementById('myButton'); const handler = new ButtonHandler(button);

In this example, we use the bind method to make sure this in the handleClick method points to ButtonHandler instance.

Common Errors and Debugging Tips

Common errors when dealing with this pointer in closures include:

• Forgot to use arrow functions or bind methods, causing this to point to the global object.
• In strict mode, this in the internal function will be undefined instead of a global object.

Debugging Tips:

• Use console.log(this) to output this value at different locations to help you understand the pointing of this .
• Use breakpoint debugging in development tools to gradually track changes in this .

Performance optimization and best practices

There are several best practices worth noting when dealing with this pointer in closures:

• Use arrow functions : arrow functions can not only solve this pointing problem, but also make the code more concise.
• Avoid overuse of bind : Although the bind method works, overuse increases memory consumption, because a new function is created with each call.
• Keep your code readable : When using closures, make sure your code is structured and well-commented so that other developers can easily understand your intentions.

Performance comparison

Let's compare the performance of different methods:

function testArrowFunction() {
    const obj = {
        name: 'test'
    };
    const func = () => {
        console.log(this.name);
    };
    for (let i = 0; i < 1000000; i ) {
        func.call(obj);
    }
}
<p>function testBindMethod() {
const obj = {
name: &#39;test&#39;
};
function func() {
console.log(this.name);
}
const boundFunc = func.bind(obj);
for (let i = 0; i < 1000000; i ) {
boundFunc();
}
}</p><p> function testVariableMethod() {
const obj = {
name: &#39;test&#39;
};
function func() {
const self = this;
return function() {
console.log(self.name);
};
}
const innerFunc = func.call(obj);
for (let i = 0; i < 1000000; i ) {
innerFunc();
}
}</p><p> console.time(&#39;Arrow Function&#39;);
testArrowFunction();
console.timeEnd(&#39;Arrow Function&#39;);</p><p> console.time(&#39;Bind Method&#39;);
testBindMethod();
console.timeEnd(&#39;Bind Method&#39;);</p><p> console.time(&#39;Variable Method&#39;);
testVariableMethod();
console.timeEnd(&#39;Variable Method&#39;);</p>

Run this code and you will find that the performance of the arrow function is usually the best, because it does not require creating new function instances.

Stepping on pit points and thinking deeply

There are several common pitfalls to be noted when dealing with this pointer in closures:

• Limitations of arrow functions : Arrow functions cannot be used as constructors because they do not have this of their own. In scenarios where constructors are required, you need to use traditional function definitions.
• Overhead of bind method : Although bind method can effectively solve this pointing problem, it creates a new function instance, which may be a problem in performance-sensitive applications.
• Variables save this complexity : This approach, while effective, may lead to a decrease in code readability in complex code, as additional understanding of the role of variables such as self or that is required.

Think deeply:

• Choice of design patterns : When designing code, consider using design patterns such as module patterns or immediate execution of function expressions (IIFEs), which can help you better manage scope and this pointing.
• Impact of Strict Mode : In Strict Mode, this default behavior will vary, and understanding these differences can help you write more robust code.
• Function Curriculization : In some cases, function curriculization can help you better manage this pointing while improving code reusability and flexibility.

Through these methods and techniques, you can flexibly control this pointing in closures and write more efficient and easier to maintain JavaScript code. Hope this article can help you better understand and resolve this pointing problem in closures.

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