JavaScript Closures

JavaScript closure is a powerful and important concept in JavaScript. It refers to a function, even after its external (closed) function is executed, it can still access variables of external functions. Closing is a key component of JavaScript scope and functional programming.

The working principle of closure

When a function is defined inside another function, the internal function can access the variables and parameters of the external function. Even after the external function is executed, the internal function still retains the access permissions of these variables. This behavior is called closure.

Closure example

<code class="language-javascript">function 外部函数() {
    let 外部变量 = '我来自外部作用域';

    function 内部函数() {
        console.log(外部变量); // 内部函数可以访问外部变量
    }

    return 内部函数;
}

const 闭包示例 = 外部函数();
闭包示例(); // 输出： "我来自外部作用域"</code>

In this example:

is a closure, because it still retains the access of

even after the 内部函数 execution is completed. 外部函数 外部变量 The actual application of closure

Data privacy

: The closure can be used to create private variables, and these variables can only be accessed by specific functions.

The callback function

<code class="language-javascript">function 创建计数器() {
    let 计数 = 0;
    return function() {
        计数++;
        return 计数;
    };
}

const 计数器 = 创建计数器();
console.log(计数器()); // 输出： 1
console.log(计数器()); // 输出： 2
console.log(计数器()); // 输出： 3</code>

: Closed is widely used for asynchronous programming, such as event processing program or AJAX calls.

Corrihua

: Closed for function Corrihua, one of the functions returns another function with some parameters.

<code class="language-javascript">setTimeout(function() {
    console.log('这是一个使用闭包的回调函数');
}, 1000);</code>

3. The advantages of closure

The control domain control

<code class="language-javascript">function 乘法(a) {
    return function(b) {
        return a * b;
    };
}

const 乘以二 = 乘法(2);
console.log(乘以二(5)); // 输出： 10</code>

: Closure helps control the scope of variables to prevent them from polluting the global scope.

Memory efficiency

: The closure allows the memory by retaining the access permissions of specific variables to effectively use memory without creating global variables.
• The precautions for closing
• Memory leak : If it is not used properly, the closure may cause memory leak, because they retain the reference to external variables, which may prevent garbage recycling.

Performance overhead

: Over -use closure may cause performance problems due to increased memory consumption.

• Examples in the real world: Private variables
In this example:
• Variables are private, and can only be accessed or modified through the method provided by the closure.

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