Logical operators in JavaScript — how do you use them?

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Logical operators in JavaScript — how do you use them?

I don't understand how &&, ||, and ! work... both with bools and other data types. How do you use them?

&&

||

!

2 Answers
2

All values in Javascript are either "truthy" or "falsy".

a && b

a || b

Both operators will not evaluate any operands after the one the return.
If all operands don't match, it will evaluate to the last one.

!a

true

a

false

a

All values are truthy except the following, which are falsy:

false

+0

-0

NaN

""

null

undefined

document.all

It should state "All values are truthful...", since i.e. undefined is a value, but there's also the variable undefined which holds this value, by default. Also NaN is not equal to anything, not even to itself.
– Ivo Wetzel
Dec 26 '10 at 22:33

undefined

NaN

@Ivo: 1: Fixed. 2: That's irrelevant; !NaN is true, so NaN is falsy.
– SLaks
Dec 26 '10 at 22:38

!NaN

true

NaN

Still NaN == NaN is false. So it's still a special case from the above list one might think that NaN works like every other falsy value.
– Ivo Wetzel
Dec 26 '10 at 23:33

@Ivo: SLaks is only listing JavaScript's falsy values. He isn't claiming anything else about NaN or any of the other values.
– Sasha Chedygov
Dec 26 '10 at 23:40

NaN

If you want to test that both of two conditions are truthy then use &&:

if (isX && isY) { // Do something. }

If you want to test that one or both of two conditions are truthy then use ||:

if (isX || isY) { // Do something. }

The ! inverts a boolean (a truthy value becomes false and vice versa).

!

What about document.write("Cat"||"Dog")?
– DarkLightA
Dec 26 '10 at 22:27

document.write("Cat"||"Dog")

@DarkLightA: I'd suggest that you don't write code like that! It causes implicit conversions and it will confuse everyone.
– Mark Byers
Dec 26 '10 at 22:31

Okay, it was in MDC :D
– DarkLightA
Dec 26 '10 at 22:39

It shouldn't confuse anyone. Short-circuiting has been a common idiom in javascript since around 5 years ago. Crockford calls || the default operator so "Cat" || "Dog" should read cat, defaults to dog. Personally I read it as cat, otherwise dog.
– slebetman
Dec 27 '10 at 0:33

||

"Cat" || "Dog"

cat, defaults to dog

cat, otherwise dog

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