29. Logical ops and value evaluation #

Created Thursday 3 March 2022

• [ ] in vault
• [x] In brain memory

In JavaScript, the logical AND and OR have special meaning.

• OR evaluates to the first truthy value. So false || 'Sanjar' evaluates to Sanjar. If all are falsy, it returns evaluates to the last value, so false || undefined evaluates to undefined. This is because of the short circuit behavior of OR. This behavior is used at many places to set default values to variables, like so:

function f(name) {
	name = name || '<Default Name here>';
}
f(); // if nothing is passed, name = undefined, so the string is returned

• AND returns the first falsy value. It returns the last value if all variables involved are truthy. So true && 1 && 'Sanjar' evaluates to Sanjar. Also undefined && true && 'Sanjar' evaluates to undefined. This due to the short-circuit behavior of AND.

In short AND and OR are logical operators, but they don’t necessarily evaluate toBoolean values.

Note: In ES6+ functions, default values can be placed in params with the assignment operator, and OR is not needed.