L'objet global en JavaScript

In JavaScript, the global object is a key concept for all developers. It contains all globally accessible variables, functions, and objects in your program. This post seeks to explain what exactly the global object is, how it behaves in different environments, and why managing it correctly is crucial for effective, clean, and maintainable code.

What Is the Global Object?

The global object can be understood as the environment where JavaScript runs. It is the container for all global variables and functions. In the browser, the global object is window _while in Node.js it is _global. Any global variable or function declared outside of functions is accessible through the global object.

Browser

var a = 1;
console.log(window.a); // 1

Node.js

var a = 1;
console.log(global.a); // 1

Global Scope and Variables

Variables declared globally are attached to the global object. Conflicts can arise here if multiple scripts share the same global space.

Imagine you have two different scripts that both declare a variable with the same name in the global scope using var.

Script 1

var message = "Hello!";
console.log(message) // Hello!

Script 2

var message = "Goodbye!";
console.log(message); // Goodbye!

When both scripts run, they both declare a message variable in the global scope using var. The second script overwrites the value of message that was set by script 1 because both variables are stored in the same global space(window in browsers). This results in the difference in output.

It’s best to avoid this by limiting the use of global variables. Keep the scope as local as possible.

globalThis and window

Until recently, developers had to use window in the browser and global in Node.js to access the global object. However, ES2020 introduced globalThis, a unified way to access the global object across environments. globalThis standardizes access to the global object.

Global Functions and Objects

The global object also holds built-in objects and functions, such as: console, setTimeout _and _setInterval, JSON, and Math. You can access these globally without needing to reference the global object directly.

So how do we maintain best practices while maintaining clean, effective code? The global object is useful but relying on it too much can lead to issues. Here is some advice: avoid using global variables (var) to reduce the risk of name conflicts and unintended side effects and instead use local variables within functions. Use let and const.

Global Object in the Browser

In the browser, the window object is more than just a global container. It also holds the DOM (document), browser APIs (fetch), and other web-specific objects. This makes window central to any browser-based JavaScript execution.

Node.js 中的全域物件

在 Node.js 中，全域物件是 global，但它不會像瀏覽器中的 window 那樣自動向其新增變數。 Node.js 鼓勵使用模組來保持全域命名空間的乾淨和結構化。

結論

全域物件是 JavaScript 處理全域變數、函數和物件的核心。了解它在不同環境中的工作原理、謹慎使用它並遵循最佳實踐將幫助您編寫乾淨、可維護的程式碼。透過使用 let 或 const 最小化全域變量，您可以避免問題並提高您的 JavaScript 技能。

來源：

全域物件

全域物件

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