Type inference ignores explicit type annotation

[belyakov-am]

🔎 Search Terms

type inference
explicit type
narrowing
type annotation

🕗 Version & Regression Information

TS version 5.1.6

⏯ Playground Link

https://www.typescriptlang.org/play?#code/IYZwngdgxgBAZgV2gFwJYHsI1JKBBEEVAcwgFsBTCZACggC4YIEyAjCgJwEpGAFD9GVQgKAHmZtOAPhgBvAFAwlMADYVkMZBQAeyRiGQdUEYjAA+TBCpUwAvJesBueYuUQKAdxj9BwijRoOChB0FQA3Ci47GQVlOM0dDXsAchDKBN1k53jlIJDw-yQAEwo4YwoirlclAF8q6uwPYFQNdy8fIREAvNCIqNsZEWQAFVRKdARaHoKAGhgARgAGZcWuLmcG1DgYGi1dO1t7Zmso2JyYIOQEDixFhpqXOMvrrD3kADo1E2QAC2caoA

💻 Code

async function asyncAssignment(n: number): Promise<number> {
    let text: string | null = null;

    new Promise((resolve) => {
        text = 'some text';
        resolve(undefined)
    })

    await new Promise((resolve) => setTimeout(resolve, 10000));

    if (text === null) {
        return 0
    }

    return text.length;
}

🙁 Actual behavior

Typescript narrows text type to null and not to string | null

🙂 Expected behavior

Expecting to have string | null type for text before explicit type narrowing (if (text === null)) and string type after explicit type narrowing

[MartinJohns]

This is working as intended. It's partially a duplicate of #9998 (modification in closure is not considered), and the intentional behavior that assignment narrows the variable (that's why text is typed null).

You can work around this by using a type assertion: let text = null as string | null;

[TmLev]

This is working as intended.

IMHO, this is suboptimal and not the behaviour anyone expects from the explicit type annotation.

[MartinJohns]

The behaviour is what most people expect. When they assign a string value to a variable annotated string | null they don't expect it to be suddenly typed string | null, they expect it to be typed string. You're assigning null, so the variable is typed null.

The only reason this is "unexpected" is because of #9998.

[TmLev]

When they assign a string value to a variable annotated string | null they don't expect it to be suddenly typed string | null, they expect it to be typed string.

This logic should only apply to const variables, not let variables. let variables can be changed at any point in time – why initial assignment dictates the type for the whole lifetime of the variable?

I find it frustratingly unintuitive.

[MartinJohns]

Consider this example:

let text: string | null = someFunctionReturningStringNotNull();
text.substring(1);

// Later in code:
text = null;

Should this code error? You're proposing yes, most people (including me) expect no.

why initial assignment dictates the type for the whole lifetime of the variable?

It doesn't. It dictates the type until the next assignment. The assignment in OPs example is just not seen due to #9998.

---

Regardless, this has come up many times already, and the position of the team is clear on this.

[TmLev]

Should this code error?

With explicit type annotation? Yes.
Without it (inferring to string)? No.

Regardless, this has come up many times already, and the position of the team is clear on this.

Closing as duplicate?

[MartinJohns]

Without it (inferring to string)? No.

Without a type annotation the variable is typed string, and it's not possible to assign null to it.

[TmLev]

Without a type annotation the variable is typed string, and it's not possible to assign null to it.

But the code is explicitly stating possible types: string or null.

It's nice to have Typescript infer the initial type on the first assignment, but ignoring explicit annotation in cases where there are no more visible to Typescript assignments in the following code is beyond my understanding.

In my opinion, inference should never overrule explicit type annotations. Inference should help, not roadblock.

[MartinJohns]

Given my example, your suggestion would result in an error on text.substring(1) with a type annotation (because it's typed string | null), and on text = null without a type annotation (because the type of the variable is inferred to be string, not string | null). Can't win either way.

See also this comment: #51487 (comment)

This used to not be the case, and people found that behavior very annoying and inconsistent, so we changed to this other behavior which is apparently also very annoying and inconsistent.

[TmLev]

My point is: if a variable has an explicit type annotation, then this annotation should have higher priority then the inferred type of the first assignment.

In your example, text.substring(1) should be inside an explicit type narrowing if, since variable text has string | null type. There's nothing to win, it's just how Typescript works in all other scenarios (consider function handle(text: string | null)).

[MartinJohns]

In your example, text.substring(1) should be inside an explicit type narrowing if, since variable text has string | null type.

And I argue: It should be typed string, because the function always returns a string, it never returns null (return type of the function is string, not string | null). Why should I check for null, when the type system knows it can never be the case? This will surprise a lot more people.

The culprit in the end is still the missed assignments due to #9998. In all other cases this behaviour is what most people would expect (but maybe not you :-p).

But alas, we're moving in circles.

Another comment by the team: #50444 (comment)

Initialization does perform narrowing. Not doing this opens you up to stuff that just looks broken as heck; this used to not happen and everyone hated it.

[TmLev]

Why should I check for null, when the type system knows it can never be the case?

Because the code explicitly states string | null. Don't want to check for null? Remove explicit annotation. Want to widen possible types? Create a new variable.

Initialization does perform narrowing. Not doing this opens you up to stuff that just looks broken as heck; this used to not happen and everyone hated it.

I'm not saying "don't perform narrowing on initialisation". Do it, but respect the explicit type annotation – it's there for a reason.

[MartinJohns]

respect the explicit type annotation – it's there for a reason.

Yep, it is. To limit what values can be assigned to it. That's the purpose of the type annotation, and that's still happening. :-D

It's working as intended, and the suggested change would be a massive breaking change and be so unnecessarily annoying in a lot of code.

Anyway, just circling and rehashing arguments, so I'm out.

[fatcerberus]

You're proposing that I should have to write:

let x: string | null = "foo";
if (x !== null) doStringThing(x);
x = null;

Just because I might later assign it a null value. That's nonsense.

[TmLev]

You're proposing that I should have to write: ...

Write this:

const x = "foo";
doStringThing(x);
const maybeX = condition ? x : null;