If condition resolving as true when value is zero

Question

My code basically looks like this:

console.log(placeCost) //this returns 0 (the number, not string)
 if (!placeCost || placeCost == false || placeCost == "undefined" || placeCost == '') {
 console.log("no")
             }
else {console.log('yes')}

Results is "no" in the console. Why does this resolve as "true"?

because `!` negates a value, so `!0` is true, and since it is true it doesn't need to test the rest of the conditions — Patrick Evans, Sep 01 '17 at 13:53
@PatrickEvans I thought that meant that if the variable doesn't exist. — jonmrich, Sep 01 '17 at 13:54
it does, `!undefined`, `!null`, `!0` all evaluate to true. When testing variable existence you are testing against `undefined` — Patrick Evans, Sep 01 '17 at 13:54
@jonmrich if the variable doesnt exist, the engine will crash. If it exists, but is nit defined yet, it is *undefined* — Jonas Wilms, Sep 01 '17 at 13:55
also: `var a = []; a == !a`. JavaScript is kind of strange in this regard.. — Bellian, Sep 01 '17 at 13:55
@PatrickEvans I removed the `!placeCost` part and it still returns "no " — jonmrich, Sep 01 '17 at 13:56
this is because `placeCost == false` is true for `placeCost = 0`. Use `placeCost === false` to check the type — Bellian, Sep 01 '17 at 13:57
`placeCost == "undefined"` is comparing the value of `placeCost` the the string `"undefined"`. Remove the quotes to test if the the variable is an undefined variable. `placeCost == undefined` — mhatch, Sep 01 '17 at 14:03

Bellian · Accepted Answer · 2017-09-01T14:03:51.123

4

Try using the === operator and don't check for !var if you plan to accept falsy parameters. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness

if (placeCost === false || placeCost === "undefined" || placeCost === '') {
    console.log("no")
} else {
    console.log('yes')
}

edited Sep 01 '17 at 14:03

answered Sep 01 '17 at 13:59

Bellian

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davidxxx · Answer 2 · 2017-09-01T14:01:45.233

2

You can refer to comparison operators :

Equality (==)

The equality operator == converts the operands if they are not of the same type, then applies strict comparison.

So !placeCost (like its more verbose form : placeCost == false) is evaluated to true if placeCost is the 0 number as 0 is converted to the false value.

You want do a strict comparison ?
Use === that performs no conversion :

The identity operator returns true if the operands are strictly equal (see above) with no type conversion.

edited Sep 01 '17 at 14:01

answered Sep 01 '17 at 14:00

davidxxx

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for reference: https://stackoverflow.com/questions/19839952/all-falsey-values-in-javascript – AJ X. Sep 01 '17 at 14:01

score 0 · Answer 3 · answered Sep 01 '17 at 14:01

0

if placeCost is 0 then !placeCost become 1, that will leads the condition is true.

if (!placeCost) // condition true when placeCost is 0

answered Sep 01 '17 at 14:01

Deepu Reghunath

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If condition resolving as true when value is zero

3 Answers3