JavaScript check for value including 0

Question

In javascript I need to check for a value in an if statement if it exists. The thing is 0 can be one of the accepted values. so when I do

if(!val) {
   return true
}

return false

The thing is Javascript evaluates !0 = false

here are test cases what I want:

val = 0 // true
val = 91 // true
val = null // false
val = undefined = false

Basically check check for null but include 0. Not sure how to do this :/

Does this answer your question? [JavaScript check if variable exists (is defined/initialized)](https://stackoverflow.com/questions/5113374/javascript-check-if-variable-exists-is-defined-initialized) — esqew, Sep 16 '21 at 20:45
@evolutionxbox no, it isn't; null and undefined are not the same thing. (I'm not certain which one the asker really cares about here, but they're not equivalent) — Daniel Beck, Sep 16 '21 at 20:48
@DanielBeck sorry I didn't mean to imply they are. `val != null` is only false for null and undefined — evolutionxbox, Sep 16 '21 at 20:58
I see what you mean, @evolutionxbox -- you're right, that does work (it depends on one of those weird js non-strict equality edge cases though, they make me itchy :) — Daniel Beck, Sep 16 '21 at 21:19

score 0 · Answer 1 · answered Sep 16 '21 at 20:49

For type check you have:

typeOf: // note: typeof [] = object and typeof {} = object https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

Array.isArray(): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

isNaN()/Number.isNaN(): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN

Is one of those what you're looking for?

!0 = true // in js. just tried it on MDN

score 0 · Answer 2 · edited Sep 16 '21 at 21:00

0

I think you need to be checking for null & undefined in your conditional statement. That way, anything that is 0 or greater will return true. You also need to check the typeOf() for undefined because you undefined is not the value of the variable.

if (typeof(val) != 'undefined' && val != null && !(val < 0)){
    return true;
}else{
    return false;
}

edited Sep 16 '21 at 21:00

Dharman

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answered Sep 16 '21 at 20:55

Luke Poirrier

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score 0 · Answer 3 · answered Sep 16 '21 at 20:57

Check for undefined and null:

function isValNumber(val) {
  if (val === undefined || val === null) {
    return false
  }
  return true
}

console.log(isValNumber(0))
console.log(isValNumber(91))
console.log(isValNumber(null))
console.log(isValNumber(undefined))

dikuw · Answer 4 · 2021-09-17T12:25:16.383

0 is falsy so !0 is true in JavaScript, but to meet your test cases I think you want truthy values to return true, and you need to handle the special case for 0. One way to do it:

function test(val) {
  if (val || val === 0) {
    return true
  }
  return false
}

console.log(test(0));
console.log(test(91));
console.log(test(null));
console.log(test(undefined));

You could also leverage Javascript's weak equivalence operator and use != null which covers null and undefined as below:

function test(val) {
  if (val != null) {
    return true
  }
  return false
}

console.log(test(0));
console.log(test(91));
console.log(test(null));
console.log(test(undefined));

JavaScript check for value including 0

4 Answers4