Update Variable Type After a Type Narrowing

[titouandk]

🔍 Search Terms

narrowing, type, variable, redefinition

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

TypeScript should understand that the type of a variable has changed, following a type narrowing:

Playground link:
https://www.typescriptlang.org/play/?#code/KYDwDg9gTgLgBAMwK4DsDGMCWEVzAQygGdgAVCMTNASQBMAKNYAG2YDV9mlgAuOVANYoIAdxQBKPiiQBbAEbAocAN4AoAJDNg8GBSp04AXjhNWHLsADcqjQHpbJ-ER0Q4zqJhQBzOJgR4IIiJMOS0NPzh6GABPMGAIf11KGlojQ2MAIml5RQzxFQ11JP1U42KUgDpdAGUYD296cWt1AF8bdQio2PjEvRS4AEJ0uAz3Ty88gvUigAsoUTgUYBE4AFEoeah6DOoUADdOTFTy31o85raNcoMyvroKojBmTBhtiryAbQAGAF1m69KcAAcrIFFAKgRiMBdq8AU12p0QTlwZgiED8ECondaOJ8mppjA5gslit1pt6AADXYHZ7HPqnEYAEmUAJaGQp8Na7Sg2iQUFwAOsbSAA

📃 Motivating Example

TypeScript does not understand that the type of a variable has changed following a type narrowing.

The type of a variable is announced as the type with which the variable has been declared - even if its type has been refined since via type narrowing.

💻 Use Cases

What workarounds are you using in the meantime?

Creating a copy of the variable. TypeScript understands the type of the new variable.

[MartinJohns]

Duplicate of #27706.

[typescript-bot]

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