The answer
Falsy. if (undefined) is not entered, !undefined is true, and Boolean(undefined) is false. But if (!x) also catches 0, '', null, false, and NaN, so use === undefined for precise checks.
JavaScript Weakly Typed Language and undefined Behavior
Is undefined truthy or falsy in JavaScript?
More FAQs in JavaScript Weakly Typed Language and undefined Behavior
NaN. undefined coerces to NaN in numeric contexts, and any arithmetic with NaN produces NaN, which propagates silently through calculations.
'Value: undefined'. The + operator favors string concatenation, so undefined is coerced to the string 'undefined' and concatenated.
Yes. undefined == null is true, a special case in the spec. But undefined === null is false. They are only loosely equal to each other, not to any other value.
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