What is the best way to check if a variable is set?

Question

In my opinion Javascript has quite a few weird quirks. Here is one of them

var a;
!a //true, a is not set
a = null
!a //true, a is not set
a = 1
!a //false, a is set!
a = 0
!a//true, a is not set!

All of these values i find to be quite reasonable, except for the case where a = 0, this is just plain wrong to me. Is there any reasonable way of circumventing this issue without having to add to bulk to my code?

[`undefined`](https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/undefined) ? — Touki, Apr 16 '13 at 06:46

score 1 · Answer 1 · edited May 23 '17 at 12:12

1

check it using typeof

if(typeof(a) != "undefined") {
     //code goes here
}

Here are some related questions.

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Test if a variable is defined in javascript?

edited May 23 '17 at 12:12

Community

1
1

answered Apr 16 '13 at 06:46

Mithun Satheesh

27,240
14
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3

As `typeof` is an operator and not a function, you don't need the brackets actually. – insertusernamehere Apr 16 '13 at 06:51

score 0 · Answer 2 · answered Apr 16 '13 at 06:55

0

if (typeof a !="undefined")
{
 //write your code here
}

answered Apr 16 '13 at 06:55

Vijendra Singh

628
3
13

What is the best way to check if a variable is set?

2 Answers2