How to use javascript closures

Usage of JavaScript Closures

JavaScript is a widely used programming language that can be used to develop different types of applications. Among them, closure is an important feature of JavaScript, which plays an important role in JavaScript programming. This article will introduce the use of JavaScript closures in detail.

What is JavaScript closure?

Before introducing the usage of JavaScript closures, we need to first understand what JavaScript closures are. Simply put, a closure refers to a function defined inside a function, and the function can access the variables and parameters of the external function. This internal function forms a closed environment so that the variables and parameters in the external function will not be destroyed after the function is executed.

Usage of JavaScript closures

Below, we will introduce some usage of JavaScript closures:

1. Encapsulating private variables

By using closures, we can encapsulate variables and functions in a private scope, thereby creating a function similar to a private variable. In this way, external code cannot directly access these variables and functions, thus ensuring the safety and stability of the program.

For example, we can define a closure function to encapsulate a variable:

function counter() {
    var count = 0;
    return function() {
        count++;
        console.log(count);
    }
}

var c = counter();
c(); // 输出1
c(); // 输出2
c(); // 输出3

In the above code, we define a closure function counter, and define it inside the closure function A variable count and returns a function that can access the external variable count. In this way, we create a private variable count that cannot be accessed directly by external code. The value of the counter can only be accessed by calling the returned function.

2. Caching the results of the function

In some time-consuming calculations, we hope to cache the results of the function to improve the execution efficiency of the function. By using a closure, we can store the result in a variable inside the closure and return that value directly the next time the function is called, thus avoiding double calculations.

For example, we can define a closure function to calculate the Fibonacci sequence:

function fibonacci() {
    var cache = {};
    return function(n) {
        if (n in cache) {
            return cache[n];
        }
        if (n <= 1) {
            return n;
        }
        var result = fibonacci(n-1) + fibonacci(n-2);
        cache[n] = result;
        return result;
    }
}

var fib = fibonacci();
console.log(fib(10)); // 输出55
console.log(fib(20)); // 输出6765
console.log(fib(30)); // 输出832040

In the above code, we define a closure function fibonacci to calculate Fibonacci. That sequence of numbers. A cache object is defined inside the function to save the cached result. Each time the function is called, it first checks whether the result has been cached. If so, the value is returned directly; otherwise, the value is calculated and the result is stored in the cache. , and finally returns the result.

3. Avoid scope problems in loops

In for loops, because the variable scope in JavaScript is at the function level, that is, the scope defined in the loop Variables are visible throughout the function. This leads to the problem of variable sharing within closures when using anonymous functions in loops. By using closures we can avoid this problem.

For example, we can define a closure function to handle click events in the loop:

for (var i = 1; i <= 5; i++) {
    (function(j) {
        document.getElementById('btn-'+j).addEventListener('click', function() {
            console.log(j);
        });
    })(i);
}

In the above code, we use an anonymous function that is called immediately to create a closure . In each loop, the variable i is passed to the function as a parameter, and a new variable j is created to hold the value of the parameter. In this way, the closure created in each loop will save the value of j in its own scope, avoiding the problem of variable sharing.

Summary

Closure is an important feature in JavaScript. It allows functions to create a private scope and save variables and functions in this scope, thereby improving program security. sex and stability. In this article, we introduced the use of JavaScript closures, including encapsulating private variables, caching the results of functions, and avoiding scope problems in loops. I hope this article can help you understand JavaScript closures.

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