来自 MDN 和书籍的 JavaScript `this` 示例不起作用......为什么？

Question

I'm trying to understand JS this because it has been too confusing to me for too long, and no one has given me an explanation I can comprehend so far. I went to MDN and tried a simple code example from their documentation. The code straight copied from their page is as follows (console logs are my only addition):

var obj = {a: 'Custom'}
var a = 'Global'
function whatsThis () {
    return this.a
}

console.log(whatsThis())
console.log(whatsThis.call(obj))
console.log(whatsThis.apply(obj))

According to MDN, the output should be:

Global
Custom
Custom

Instead, I get

undefined
Custom
Custom

Incredulous, I tried an example from the somewhat-famous You Don't Know JS series, chapter 2 of book 2, regarding this . I tried the following example, verbatim:

function foo() {
    console.log( this.a );
}

var a = 2;

foo();

The book says the output should be 2 , but I get undefined .

This is exactly why I want to learn more about this , but even books and MDN straight code examples are failing for me, making it very difficult to get my mind wrapped around a core concept of JS. Please help me set this straight in my head.

Answer 1

Variables declared with var in a Node.js module don't end up on the global object – modules have their own scope. You can put the properties you want to read on the global object explicitly:

global.a = 'Global';

global.a = 2;

(In browsers, it's window instead of global , or you can use the new portable globalThis for both.)

However, you won't need to worry about this behaviour too much. In strict mode , which you should always use (and which will be applied automatically inside new features like classes and ES modules), the this of a plain function call like whatsThis() is just undefined .

 'use strict'; function whatsThis() { console.log('this is ' + this); } let obj = { whatsThis, }; whatsThis(); whatsThis.call('a string'); obj.whatsThis();

The global object in Node should essentially never be used, and declaring variables with var at the top level in browser scripts is similarly bad practice.