CODEWITHSUNDEEP
javascriptnode.jsbind
ofey
ofey

Reputation: 3347

.bind() example incorrect output

I am learning about .bind() from the example on MDN here, the only changes that I've made to this example is where I added the console.log()'s.

this.x = 9;    // this refers to global "window" object here in the browser
var module = {
  x: 81,
  getX: function() { return this.x; }
};

console.log(module.getX()); // 81

var retrieveX = module.getX;
console.log(retrieveX());   
// returns 9 - The function gets invoked at the global scope

// Create a new function with 'this' bound to module
// New programmers might confuse the
// global var x with module's property x
var boundGetX = retrieveX.bind(module);
console.log(boundGetX()); // 81

Running this using Node I get,

$ node bind.js
81
undefined
81

The 'undefined' output should be '9'.

Why am I not getting that output?

Here's a screenshot from my attempt with Node:

enter image description here

Upvotes: 2

Views: 89

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1075039

Running this using Node I get...

Because in NodeJS, your seemingly-top-level code isn't running at global scope and in it, this doesn't refer to the global object. All NodeJS code runs within a NodeJS module, which is not global scope. Since this doesn't refer to the global object, this.x = 9; doesn't create a property called x on it, and so when the unbound function is run with the wrong this (with this referring to the global object), it gets undefined instead of 9 for this.x property since there is no global property called x.

If you ran that code in a browser, it would do what MDN says it does. Alternately, you can change this.x = 9; to global.x = 9; and it will on NodeJS as well, because in NodeJS, global is a reference to the global object (just like this at global scope in browsers, and just like the window identifier in browsers).

You'll find that a lot of code examples which assume they're run at global scope will behave slightly differently when run in NodeJS, for this very reason. For instance, var n; at global scope creates a property on the global object, but var n; at the top-level of a NodeJS module just creates a variable scoped to that module, not a global.

Upvotes: 4

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