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.

Comienza con el TypeScript Playground enlazado y reproduce la diferencia entre let xx: number y let yy!: number. Rastrea cómo el análisis del uso antes de la asignación afecta al tipo reportado y compara el manejo existente de typescript-eslint enlazado en el issue. Se considera terminado cuando la primera variable se muestra como number | undefined sin romper el diagnóstico ni el caso con aserción de no nulo.