ts ignores type annotation and generate wrong type inference

[chenxinhang12345]

🔎 Search Terms

wrong type inference

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about ?

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240313#code/DYUwLgBAhgXBDOYBOBLAdgcwgHwmgrsMBALx6HADcAUAMYD2aiEARqRANoCMANAEw8AzAF0aLAHQAzekgCiUWgAsAFMtoBKUgD4IAb2oRo7AMrJ0GNepoBfK9RSTlUCAEISZAkXX7DhgPR+RijweCAAbiBIEOgQYIrBrMD0tADWBr4MTPSg4kkWUHbW1EA

💻 Code

let a: string | null = null;
const b = [1,2,3];
b.forEach((c) => {
  a = String(c);
});
if(a !== null){
    // a is never in this block
    console.log(a);
}

🙁 Actual behavior

variable a is inferred as null in the if block, causing the variable a in the if block to be type never.

🙂 Expected behavior

a should be inferred as string

Additional information about the issue

let a: string | null = null as (string | null);
const b = [1,2,3];
b.forEach((c) => {
  a = String(c);
});
if(a !== null){
    // a is never in this block
    console.log(a);
}

This workaround magically works.

[MartinJohns]

The not recognized assignment in the callback is a duplicate of #9998.

That your type annotation is not used is working as intended. Assigning a value narrows the variable. The type annotation limits what values can be assigned to it.

[typescript-bot]

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