Javascript || operator

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There is already an accepted answer, but I like to mention, that the OR-Operator is also called Default-Operator, because it doesn’t return a boolean, but instead the left or right hand argument.

Same goes for the AND-Operator, which is also called guard-Operator.

Check out crockford’s Survey of the JavaScript Programming Language for more details:

The && operator is commonly called logical and. It can also be called guard. If the first operand is false, null, undefined, “” (the empty string), or the number 0 then it returns the first operand. Otherwise, it returns the second operand. This provides a convenient way to write a null-check:

var value = p && p.name; /* The name value will
only be retrieved from p if p has a value, avoiding an error. */

The || operator is commonly called logical or. It can also be called default. If the first operand is false, null, undefined, “” (the empty string), or the number 0, then it returns the second operand. Otherwise, it returns the first operand. This provides a convenient way to specify default values:

value = v || 10; /* Use the value of v, but if v
doesn’t have a value, use 10 instead. */

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