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

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All values in JavaScript are either “truthy” or “falsy”.

  • a && b evaluates to the first falsy operand,
  • a || b evaluates to the first truthy operand.

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

  • !a evaluates to true if a is falsy and false if a is truthy.

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

  • null
  • undefined
  • false
  • +0
  • -0
  • NaN
  • 0n
  • ""
  • document.all
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6 Comments

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: 1: Fixed. 2: That's irrelevant; !NaN is true, so NaN is falsy.
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: SLaks is only listing JavaScript's falsy values. He isn't claiming anything else about NaN or any of the other values.
@Pointy - It's a quirk of web history. :-) Early on, document.all was a Microsoft-ism and IE didn't support the fledgling DOM standard, so code would do if (document.all) { /* MS-specific code */ } else { /* DOM standard code */ }. Time moved on and A) MS implemented DOM standards, and B) Other browsers added document.all to support code that relied on it unconditionally. But the code in those pages didn't move on. To avoid having code written like that taking the Microsoft-only branch, Chrome and Firefox made document.all falsy. Eventually so did IE. It's been standardized now. :-D
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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).

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3 Comments

What about 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.
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.

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