console.log printing 'undefined' when calling this on a variable

Question

I'm new to JavaScript, I can't find a clear answer to what's going on here:

function bike() {
  console.log(this.name);
}

var name = "John";
var obj1 = {
  name: "Sam",
  bike: bike
};
var obj2 = {
  name: "Paul",
  bike: bike
};

bike(); // undefined
obj1.bike(); // Sam
obj2.bike(); // Paul

I don't understand why it's printing 'undefined' on the terminal instead of 'John'

Does this answer your question? [How does the "this" keyword work?](https://stackoverflow.com/questions/3127429/how-does-the-this-keyword-work) — ASDFGerte, Dec 10 '19 at 05:34
@DevsiOdedra any idea why it would show undefined when I run it on my terminal? — beginCS, Dec 10 '19 at 05:35
Please also note, that in a browser context, you collide with `window.name`. You can write to the property, but it is a setter, that autoconverts to string. Just as a side note. — ASDFGerte, Dec 10 '19 at 05:49
it prints john, because the snippet is not in strict mode (undefined/null this gets converted to window), and global variables get added to window. It **should** throw, "cannot read property `name` of `undefined`". I don't know what your terminal is doing. — ASDFGerte, Dec 10 '19 at 05:55

Purushoth.Kesav · Accepted Answer · 2019-12-10T05:51:09.233

1

Nothing wrong with this Behaviour,

function bike() {
  console.log(this.name);
}

var name = "John";
var obj1 = {
  name: "Sam",
  bike: bike
};
var obj2 = {
  name: "Paul",
  bike: bike
};

bike(); // undefined here it refers to (**this**).name
        //if you run the same in browser console 
        // you will get jhon
        // but in node there is no window object
        // that's y you are getting undefined in node terminal

obj1.bike(); // Sam
obj2.bike(); // Paul

Plz refer the link ASDFGerte suggested for more

edited Dec 10 '19 at 05:51

answered Dec 10 '19 at 05:40

Purushoth.Kesav

615
7
16

`this` defaults to the global `window` object when not called from an object method. – Barmar Dec 10 '19 at 05:41

score -2 · Answer 2 · answered Dec 10 '19 at 05:38

-2

it's Simple you just write

window.name

window.variableName it is used to print global variable

answered Dec 10 '19 at 05:38

dhruvin prajapati

184
1
3
15

`this` defaults to `window` if you're not in an object method. – Barmar Dec 10 '19 at 05:41

console.log printing 'undefined' when calling this on a variable

2 Answers2