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, it 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:

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