TypeScript: check of `unknown` argument - are `undefined` and `null` different cases?

Question

Code inspection (IntelliJ and GitHub) says that the following code can be simplified:

type Something = string | number | boolean | object
function isSomething(a: unknown): a is Something {
    return a != undefined && a != null;
}

because a != null is always true. Why is that? AFAIK undefined and null are different values.

null == undefined is true. If you really need to distinguish undefined and null, use the === and !== operators. — Bbrk24
– Bbrk24, Commented Oct 28, 2022 at 13:16

k.tten · Accepted Answer · 2022-10-28 13:00:53Z

3

a != undefined is false if a is null. That means if a is null, the second condition will never run, since the && operator will short-circuit.

console.log(null == undefined); // true
console.log(null != undefined); // false

answered Oct 28, 2022 at 13:00

k.tten

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