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.

リンクされている TypeScript Playground から始め、let xx: number と let yy!: number の違いを再現してください。代入前使用の解析が報告される型にどのような影響を与えるかを追跡し、issue にリンクされている既存の typescript-eslint の処理と比較してください。完了条件は、診断や non-null アサーションされたケースを壊すことなく、最初の変数が number | undefined として公開されることです。