Undeclared object variable showing undefined instead of ReferenceError

Question

var obj = {a : 2};
console.log(obj.b); //prints undefined
console.log(b);      //ReferenceError

Here both obj.b and b are not defined. Can anyone explain the reason behind the different outputs?

obj.b is a property of obj, which doesn't exist (hence undefined), b is a variable that doesn't exist (hence reference error) — Lixus, Jul 08 '17 at 16:44
Possible duplicate of [How does variable assignment work in JavaScript?](https://stackoverflow.com/questions/509579/how-does-variable-assignment-work-in-javascript) — Ian, Jul 08 '17 at 16:45
You should take a look around, plenty of answers such as [this one](https://stackoverflow.com/questions/509579/how-does-variable-assignment-work-in-javascript) — Ian, Jul 08 '17 at 16:45
@Ian I checked that answer before posting this question. That answer never really explained the different outputs. — Priyanjit, Jul 08 '17 at 16:59

score 1 · Accepted Answer · answered Jul 08 '17 at 16:44

1

The first is a missing property, the second is a missing variable.

See also the difference here:

console.log(window.b);
console.log(b);

answered Jul 08 '17 at 16:44

trincot

score 0 · Answer 2 · edited Jul 08 '17 at 16:45

0

obj.b is undefined cause he don't exist in your object and var b does not exist too.

edited Jul 08 '17 at 16:45

Mr. Alien

answered Jul 08 '17 at 16:43

Kirino

score 0 · Answer 3 · answered Jul 08 '17 at 16:57

0

You can make the first part work by this

var obj = {a : 2};
console.log(obj.a); //Your property `b` was not defined or you typed `b` instead of `a`

..and the second part by this

var b = 3;
console.log(b);// `b` was not defined at all

Explanations in the comments

answered Jul 08 '17 at 16:57

James Poulose

I intentionally typed b in place of a. I was trying to find out why obj.b and b behaved differently. – Priyanjit Jul 08 '17 at 17:04

3 Answers3