raise a type error on attempt to strict-equal-compare a value of string | null to undefined

[zpdDG4gta8XKpMCd]

the following examples are almost certainly bugs:

declare var one: string | undefined;
// expected a type error: Operator '===' cannot be applied to types 'string | undefined' and 'null'
// actual no problem
if (one === null) { 
   // unreachable, because one can't ever be null, undefined was likely meant instead
}

declare var another: string | null;
// expected a type error: Operator '===' cannot be applied to types 'string | null' and 'undefined'
// actual no problem
if (another === undefined) {
   // unreachable, because one can't ever be undefined, null was likely meant instead
}

consider raising a type error for the above situations

[RyanCavanaugh]

Interested in your thoughts here. We special-cased comparisons against null and undefined specifically for defensive checks:

// TypeScript callers: Don't give me 'undefined'
function fn(x: string) {
  // JS callers: Read the docs please
  if (x === undefined) throw new Error('x cannot be undefined');
}

I think that part is non-negotiable -- it should be "cheap" to check for those sorts of errors, by some means or another.

Checking a null against string | undefined does feel really suspicious to me. Seems like once you've got one sentinel non-value in there, looking for the other is not what you meant. I could see C# programmers especially writing code like this

function fn(x?: string) {
  if (x === null) { x = "some default" } // This is how it works, right?
}

One concern I have is that strictNullChecks disallows the assertion you'd want to write if you were doing this on purpose:

if (another === <string>undefined) { // Error, can't convert undefined to string

Perhaps a type assertion from null or undefined to any type should always succeed.

[zpdDG4gta8XKpMCd]

i am not questioning #11920 here, this is what i've been through today though, i had:

interface Data { value: number | null; }
declare var data: Data;
if (data.value === null) {
   // do things
}

then i refactored Data to

interface Data { value: number | undefined; }

and

if (data.value === null) {
   // do things
}

hasn't been caught, bang! a problem

so it wasn't a silly comparison by choice, just something vulnerable to refactoring that typescript can't see

[nippur72]

experienced the same problem today: I refactored a number|null to number|undefined and my code stopped to work because there were ===null checks still left behind.

[RyanCavanaugh]

I really want some strictNullChecks real-world projects so we could test this stuff out

[zpdDG4gta8XKpMCd]

i have one, willing to test it

[jfirebaugh]

Ironically, I work on a codebase where we're incrementally enabling strictNullChecks and was bit by this.

To support strictNullChecks , the following change was made:

- let applyMemoryInitializer: () => void;
+ let applyMemoryInitializer: (() => void) | null = null;

A later condition was overlooked, and became always-true.

if (applyMemoryInitializer !== undefined) {
   ...
}

[monsanto]

@RyanCavanaugh It's been several years, is more feedback still desired? This something I've been bit by in the past and might have gotten bit by just now, if I didn't recall TypeScript's behavior in this situation.

Real simple case. I had some code that explicitly returned null, but I was thinking about code-golfing it using the ?. operator, and that would have returned undefined instead. It's easy to make this kind of mistake and TypeScript otherwise encourages you to lean on it for always true/always false comparisons.