Overwriting an unknown variable does not change the type

[midgleyc]

Bug Report

🔎 Search Terms

unknown, overwrite, replace

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about common bugs that aren't bugs

⏯ Playground Link

Playground link with relevant code

💻 Code

import {strict as assert} from 'assert';

async function overwrite(err: unknown) {
  err = 'hello';
  assert(err.startsWith('h'));
  // err is 'unknown' but should be 'string'
}

🙁 Actual behavior

Errors because err is type unknown.

🙂 Expected behavior

No error because err is a string by that point.

Motivating example

I am reading off .error from an Angular HttpErrorResponse object. error is any, and can be at least string, JSON or Blob. I set it to "unknown" for type safety.

async function dealWith(err: unknown) {
  if (err instanceof Blob) {
    const isJson = err.type.endsWith('json');
    err = await err.text();
    if (isJson) {
      err = JSON.parse(err);
    }
  }
  // code that checks whether err is string or JSON and deals with it
}

[MartinJohns]

See the comments on #42687.

Duplicate of #27706.

[midgleyc]

Thanks!

I note that the most general case #36579 has been marked as "Working as Intended". Most of the other tickets are still in discussion mode. Is narrowing unknown types on assignment a possibility? I'm not really concerned about other types: for the most part, they're much easier to work with.

[typescript-bot]

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