The OR Operator (JS Demystified)

Question

Classical Meaning:

alert(true || true) // true
alert(true || false) // true
alert(false || true) // true
alert(false || false) // false

JS Quirks:

1. Reads from Left to Right
2. Convert Operand to Boolean (any value type)
3. Returns the First Truthy (original value)
4. Returns last value if all operands are falsy.

Examples:

alert(1 || 0) // 1
alert(1 || 2 || 3) // 1 (corrected)
alert(alert(1) || 2) // 2
alert(0 || null || undefined) // undefined

Answer 1

0 is false, everything else is true.

null is nothing.

Answer 2

This is called lazy short-circuit evaluation*.

first_one_that_is_true = a || b || c || d;
first_one_that_is_false = e && f && g && h;

Behavior notes:
alert(1 || 2 || 3) actually returns 1. (edit: fixed)
alert(alert() || ...) returns the next value because alert itself returns 'undefined' (this evaluates as false). (edit:example changed; left for visitor info)

* Often used to set default values, people will put one final 'guaranteed' truthy/falsy at the end to check for a default condition.

Answer 3

Thanks @Kirk was a typo mistake....👍

Answer 4

Hi @America Perez

Answer 5

What was the reason of changing "alert()" to "alert(1)" if both return undefined?

Answer 6

0 has it rough with boolean operators
same behavior is in Py

Answer 7

Hello