Undefined variable in Javascript: difference between typeof and not exists

Question

I don't understand the difference between these two ways of checking if a variable is undefined:

if (typeof res.locals.user.hashnodes === 'undefined') {
        morphemes = 0;
}

and

if (!res.locals.user.hashnodes) {
       morphemes = 0;
}

For me, only the second option works, the first one doesn't. Anyone knows why?

(res.locals.user are the user settings I pass in my Node Js app).

Thank you!

Most likely because `res.locals.user.hashnodes` is not `undefined`, but a different kind of falsy value (e. g. `null`, `false` or `0`). — The Paramagnetic Croissant, May 25 '14 at 14:14
would ! work on an undefined one? or I need to make two checks then if I want to cover all the possibilities? — Aerodynamika, May 25 '14 at 14:15
Are you getting a JavaScript error and if so what is it? Or is the logic not functioning as you expect? — Greg Burghardt, May 25 '14 at 14:15
@deemeetree Yes, it would, since `undefined` is a falsy value. — The Paramagnetic Croissant, May 25 '14 at 14:22

score 1 · Answer 1 · answered May 25 '14 at 14:14

1

JavaScript has the so-called falsey variables, this means that !res.locals.user.hashnodes evaluates to undefined, 0, false and a few more. Here's some more info on falsey and truthy variables.

http://www.sitepoint.com/javascript-truthy-falsy/

answered May 25 '14 at 14:14

Jon Snow

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score 1 · Answer 2 · answered May 25 '14 at 14:15

1

The second will set morphemes = 0 if res.locals.user.hashnodes is undefined or false or null or 0

where as the fist will only do so it it is undefined

answered May 25 '14 at 14:15

andrew

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score 1 · Accepted Answer · answered May 25 '14 at 14:15

if (!res.locals.user.hashnodes) will also check for existing yet false values. 0, false, '', null, 'undefined', [] will all evaluate to true if you only use !.

(Here's a more thorough list of what evaluates to what)

So what value does your local hold?

console.log(typeof res.locals.user.hashnodes)

If it's anything else than 'undefined', there's your answer

Undefined variable in Javascript: difference between typeof and not exists

3 Answers3