How do I test if a variable in Javascript is initialized?

Question

How do I test if a variable in my javascript code is initialized?

This test should return false for

var foo;

and true for

var foo = 5;

if (foo == null)? or do you need to handle the case that it has been initialized to null? — Aidanc
– Aidanc, Commented Apr 16, 2012 at 20:59
possible duplicate of JavaScript check if variable exists - Which method is better? — James Montagne
– James Montagne, Commented Apr 16, 2012 at 21:02

Pointy · Accepted Answer · 2012-04-16 20:59:06Z

13

if (foo === undefined) { /* not initialized */ }

or for the paranoid

if (foo === (void) 0)

This is the sort of thing that you can test right in your JavaScript console. Just declare a variable and then use it in (well, as) an expression. What the console prints is a good hint to what you need to do.

answered Apr 16, 2012 at 20:59

Pointy

415k62 gold badges600 silver badges631 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Woody · Accepted Answer · 2012-04-16 21:21:11Z

7

Using the typeof operator you can use the following test:

if (typeof foo !== 'undefined') {
    // foo has been set to 5
}
else {
    // foo has not been set
}

I find the jQuery fundamentals JavaScript Basics chapter really useful.

I hope this helps.

answered Apr 16, 2012 at 21:21

Woody

3732 silver badges9 bronze badges

2 Comments

f2lollpll Over a year ago

This works for functions as well. Comparing a non-initialized function with null (as in the answer) will throw an exception.

seren Sep 25 at 12:43

FYI, the jQuery Javascript 101 section was deleted (see github.com/jquery/learn.jquery.com/issues/117). It can still be read in the github history: github.com/jquery/learn.jquery.com/tree/…

Collectives™ on Stack Overflow

How do I test if a variable in Javascript is initialized?

2 Answers 2

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related