How to execute javascript code once

JavaScript is a powerful programming language that can be used to create interactive web pages and applications. In some cases, we may only need to execute JavaScript code once, rather than repeatedly throughout the entire web page or application life cycle. This article will show you how to execute code once in JavaScript.

One-time execution in JavaScript can be achieved through the following three methods:

1. Immediately Invoked Function Expression (IIFE)

IIFE is an anonymous function that can be called immediately. It calls itself immediately after being defined and executes it only once. This approach uses closures and therefore avoids the use of global variables. Here is an example:

(function() {
  console.log('这个代码将只执行一次。');
})();

In this example, we define an anonymous function and call it immediately. Since this function is called immediately, it will only be executed once. You can put the code to be executed inside the function body.

2. Self-executing anonymous anonymous function

Similar to IIFE, this method also uses self-executing anonymous functions. However, in this case, we assign the anonymous function to a variable so that it can be referenced as the function that needs to be executed. Here is an example:

var runOnce = function() {
  console.log('这个代码将只执行一次。');
}
runOnce();

In this example, we define an anonymous function and assign it to a variable called "runOnce". After a function has been assigned a value, we can call it like any other function.

3. Determine whether the variable has been initialized

This method involves defining a global variable and using a Boolean value to determine whether it has been initialized. The code is executed only once if the variable is not initialized. Here is an example:

if (!window.alreadyRun) {
  console.log('这个代码将只执行一次。');
  window.alreadyRun = true;
}

In this example, we check if the global variable "window.alreadyRun" is set to "true". If the variable is not set, the code is executed and the variable is set to "true". Since the variable is defined on the global object, it will be visible throughout the program.

Summary

In this article, we introduced three ways to execute code once in Javascript. Both immediate execution function expressions and self-executing anonymous functions use self-executing anonymous functions to avoid the use of global variables. The method of checking whether a variable has been initialized is suitable for situations where state needs to be saved throughout the program life cycle. Which method you choose depends on your specific needs.

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