Different handling of 'this' in Node.js and Browser

Question

I have Node.js v8.10.0 locally installed. I wrote a simple script to play with 'this':

var x = 1;

var fn = function (a) {
     var x = a;

     console.log(`local x = ${x}`);
     console.log(`global x = ${this.x}`);
}

fn(10);

When I execute script via Node.js I get following result:

local x = 10

global x = undefined

When I execute script in Chrome I get the following result:

local x = 10

global x = 1

Could you please explain to me, why Node.js doesn't see x in global scope?

Answer 1

Could you please explain to me, why Node.js doesn't see x in global scope?

It does, if you run it in Node console. If you run it in as a file, x is in the file's scope , not global scope.

By the way, in Node, you can use global to explicitly see the global scope, just like you'd use window in a browser. Thus,

console.log(global == this)

will give you two different answers depending on whether you run it in a file or in a console.

Also, try to migrate to let and const . This is extra confusing because var behaves differently in global scope and elsewhere. In console and in browser, your outer var x is in global scope, so it defines a global variable ( window.x and global.x ). In a Node file, var x is not in a global scope, so it does what it normally does when not in global scope: defines a local variable x (not this.x , not global.x , just x ). Thus, you have two local variables, the inner one shadowing the outer one, which makes the outer one inaccessible. Meanwhile, this.x has never been defined.

Answer 2

In chrome this is a object of Window as if you do this.constructor.name you get Window as a constructor name so while accessing this.x it will look for the global variable x and does not reference to the function scope.

 var x = 1; var fn = function (a) { var x = a; console.log(`local x = ${x}`); console.log('Constructor ', this.constructor.name); console.log(`global x = ${this.x}`); } fn(10); 

However, in NodeJS, this will always refer to the function prototype (not the global scope). Thus, you do not have any value x associated with the prototype of the function so it gives you undefined .