Narrowing on await

[reverofevil]

Bug Report

🔎 Search Terms

narrowing await

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type System Behavior

⏯ Playground Link

Playground link with relevant code

💻 Code

let x: null | string = 'hello';
(async () => {
    if (typeof x !== 'string') {
        return;
    }
    await new Promise((resolve) => {
        x = null;
        setTimeout(resolve, 0);
    });
    // Cannot read property 'toString' of null
    console.log(x.toString());
})();

🙁 Actual behavior

x is narrowed to string, but await should have either extended it back to null | string, or narrowed it in the parameter of Promise constructor. toString gets called on null. In this way arbitrary type punning is possible.

🙂 Expected behavior

Code shouldn't have compiled.

[whzx5byb]

Even synchronous code can still produce the error.

let x: null | string = 'hello';
(() => {
    if (typeof x !== 'string') {
        return;
    }

    function fn() {
        x = null;
    }
    fn();
    
    // Cannot read property 'toString' of null
    console.log(x.toString());
})();

Because TS can't analyse every function to check if they change anything, so it treats every function as having no side-effect.
Related: #9998

[reverofevil]

Actually TS can analyze every function, by pure annotation. It's discussed in linked thread, and it's what tools like webpack use to make tree shaking possible.

[whzx5byb]

Actually TS can analyze every function, by pure annotation. It's discussed in linked thread, and it's what tools like webpack use to make tree shaking possible.

It is still in discussion. I guess you might be interested in #7770.

[reverofevil]

@whzx5byb Thank you! I would have spent quite some time looking for these links.

[MartinJohns]

@polkovnikov-ph Basically any assignment via closure is missed by the compiler, due to #9998. Doesn't matter if async or sync.

#11498 is also a relevant issue to follow, although there is no progress at all.

[reverofevil]

What a nice joke! Thank you!

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.