Logical operators in JavaScript — how do you use them?

Question

I don't understand how &&, ||, and ! work... both with bools and other data types. How do you use them?

Answer 1

All values in JavaScript are either “truthy” or “falsy”.

• a && b evaluates to the first falsy operand,
• a || b evaluates to the first truthy operand.

Both operators will not evaluate any operands after the one the return. If all operands don’t match, they will evaluate to the last one.

• !a evaluates to true if a is falsy and false if a is truthy.

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All values are truthy except the following, which are falsy:

• null
• undefined
• false
• +0
• -0
• NaN
• 0n
• ""
• document.all