让TS抱怨与非空字段的空比较？

Question

Is there a way to get TS to complain about this? I have strictNullChecks on

const foo: string = 'asdf';

if (foo !== null) {
    console.log(foo);
}

If, for instance, I change that condition to:

if (foo !== 42) {

I get:

This condition will always return 'true' since the types 'string' and 'number' have no overlap.

Answer 1

You can't cause an error with === because TS has special carve-outs that make comparison with null / undefined always allowed.

If you're willing to use a helper function, you can use a conditional type to ensure that null is a possible value of the tested expression:

function isNull<T>(x: T): null extends T ? boolean: void {
   return (x === null) as any;
}

// Error
declare const a: string;
if (isNull(a)) { }

// OK
declare const b: string | null;
if (isNull(b)) { }