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

undefined

undefined

NaN

!NaN

true

NaN

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).

!

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

||

"Cat" || "Dog"

cat, defaults to dog

cat, otherwise dog

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