JavaScript中全局变量与window变量的区别及存储位置咨询

Great question—this is a super common point of confusion when transitioning from ES5 to modern JavaScript, so let’s break it down nice and clearly.

1. var声明的全局变量：直接绑定到window对象 #

In the browser environment, pre-ES6 JavaScript tied the global scope directly to the window object. When you declare a global variable with var, it automatically becomes a property of the window object:

var a = 1;
alert(window.a); // 弹出 '1'

This is legacy behavior from early JS, where the global context and the browser’s top-level window object were essentially one and the same.

2. let/const声明的全局变量：独立于window #

ES6 introduced let and const to fix many of var’s odd behaviors, and one key change is how global variables are stored. When you declare a global variable with let or const, it does not become a property of the window object:

let b = 1;
alert(window.b); // 弹出 'undefined'

So where does b live? It’s stored in the declarative environment record of the global scope. Modern JavaScript’s global scope has a split environment record with two parts:

• The object environment record: Linked directly to the window object, this holds all var-declared global variables and built-in global properties (like window.alert).
• The declarative environment record: A separate, isolated container that holds let/const-declared global variables and class declarations. It’s part of the global scope, but you can’t access it via the window object.

You can confirm b is still in the global scope by accessing it directly:

console.log(b); // 输出 1

3. 存在两个独立的全局命名空间吗？ #

Nope, there aren’t two distinct global namespaces. Instead, the global scope now uses a split environment record to manage different types of declarations. Think of it as one global context with two "storage bins": one tied to window for legacy code, and an isolated one for modern ES6 declarations. They’re part of the same global scope—just stored differently.

A quick side note for Node.js users: The browser’s window doesn’t exist here; instead, there’s a global object. Similar rules apply though: var-declared globals become properties of global, while let/const globals don’t (they live in the module’s top-level scope or the global environment record, depending on context).

内容的提问来源于stack exchange，提问作者kadina