Variables that are used-before-assigned are typed without `undefined`

Bug Report

🔎 Search Terms

variable use before assign

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

let xx: number;

  xx.toFixed();
// ^?
// typeof xx === number ❌

let yy!: number;

  yy.toFixed();
// ^?
// typeof yy === number ✅

🙁 Actual behavior

TS reports the type of the used-before-assigned variable xx as number, and reports error Variable 'xx' is used before being assigned. (2454)

🙂 Expected behavior

TS reports the type of xx as number | undefined.

Additional info

This behaviour makes it difficult for us to write lint rules because when we attempt to check if the variable is undefined, the type system reports that it is not.

Examples:

• checking for unnecessary conditions based on types
  • if (xx != null) {} The types say xx is number, so != null looks like it's unnecessary.
• checking for unnecessary type assertions
  • xx! Again the types say xx is number, so the non-null assertion looks like it's unnecessary.

Example issue:

• https://github.com/typescript-eslint/typescript-eslint/issues/453
• https://github.com/typescript-eslint/typescript-eslint/issues/6640

In the past we've manually coded up logic to do used-before-assigned checks where necessary - but it's pretty gnarly code to maintain and we have to ensure that we cover this case in our implementation.

It would be great if TS reported the "correct" type here with undefined.

Inizia con il TypeScript Playground collegato e riproduci la differenza tra let xx: number e let yy!: number. Traccia il modo in cui l’analisi dell’uso prima dell’assegnazione influisce sul tipo riportato e confronta la gestione esistente di typescript-eslint collegata nell’issue. Il lavoro è completato quando la prima variabile viene esposta come number | undefined senza rompere la diagnostica o il caso con asserzione non-null.