How to change global variables in javascript

JavaScript is an object-based dynamic language with high flexibility, easy to learn and use, and is widely used in web development, game development, mobile application development and other fields. Among them, global variables are the most commonly used variable type in JS programming, and their values ​​can be accessed and modified by any code in the program. In this article, we will discuss how to change global variables in JavaScript.

1. Definition of global variables

In JavaScript, declaring a variable through the var keyword or direct assignment (without adding var) means declaring a local variable. Variables declared outside a function are global variables and can be accessed and modified throughout the program. For example:

var a = 1; //声明一个全局变量a
function test(){
  console.log(a); //输出1
}
test();

2. How to change global variables

1. Direct assignment

We can change global variables by directly assigning values ​​to global variables. value. For example:

var a = 1; //全局变量a的值为1
function test(){
  a = 2; //直接为全局变量a赋值2
  console.log(a); //输出2
}
test();
console.log(a); //输出2

After reassigning the global variable inside the function, the value of the variable will also be modified.

2. window object

The window object is a global object in JavaScript, which provides a way to access global variables and functions. Global variables can be modified through the window object. For example:

var a = 1; 
function test(){
  window.a = 2; //通过window对象修改全局变量a的值
  console.log(a); //输出2
}
test();
console.log(a); //输出2

Global variables can be accessed through the window object inside the function, and the value of the global variable can be modified directly.

3. this keyword

In the global environment, this points to the window object. Therefore, the value of global variables can also be modified through the this keyword. For example:

var a = 1; 
function test(){
  this.a = 2; //通过this关键字修改全局变量a的值
  console.log(a); //输出2
}
test();
console.log(a); //输出2

4. Global object modification

Global variables are actually properties of the global object, so the value of the global variable can be modified through the global object. In the browser environment, the global object is the window object, and in the Node.js environment, the global object is the global object. For example:

var a = 1; 
function test(){
  window.a = 2; //通过全局对象修改全局变量a的值
  console.log(a); //输出2
}
test();
console.log(a); //输出2（在浏览器环境下）

In the Node.js environment, you can modify the value of global variables through the global object:

var a = 1;
function test(){
    global.a = 2; //通过全局对象修改全局变量a的值
    console.log(a); //输出2
}
test();
console.log(a); //输出2（在Node.js环境下）

3. Notes

1. Do not add var When declaring a variable, the defined variable is a global variable, which can easily cause variable pollution and duplicate name problems, and should be avoided as much as possible.
2. When modifying global variables inside a function, you need to pay attention to whether there will be any side effects, which may affect other parts of the program.
3. Global variables are frequently used in programs and should be standardized when defining to facilitate the readability and maintainability of the program.

In short, global variables in JavaScript are one of the most commonly used variable types in programming. How to change their values ​​correctly is also one of the keys to improving programming efficiency and code quality. The above methods are the main ways to change global variables in JavaScript. We need to combine the actual development needs and practice more to better master their use.

The above is the detailed content of How to change global variables in javascript. For more information, please follow other related articles on the PHP Chinese website!