1.5 Boolean Logic

>>>

true && false

false

The && operator is the logical AND of two boolean values.

>>>

true || aMissingVariable

true

The || operator is the logical OR. The second argument is not evaluated if the first is true.

>>>

!true

false

The ! operator is the logical NOT (negation).

>>>

false || "value"

"value"

Although the && and || operators coerce their operands to boolean values, they return the value of the last operand evaluated.

>>>

null || ""

""

The || operator thereby becomes a convenient way of setting a default value. But watch out for 0...

>>>

!!0

false

Using double negation, we force boolean type coercion. The undefined, null, 0, NaN and "" values are all false. Other values are true.

>>>

!!42

true

>>>

!!""

false

>>>

!!" "

true

>>>

!!({})

true

>>>

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