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Infer Subtypes from Template Literal TypesΒ #43907

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@johnrom

Description

@johnrom

Suggestion

πŸ” Search Terms

Inference
Template Literal Types
Subtypes

βœ… Viability Checklist

My suggestion meets these guidelines:

  • 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 feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

I'd like to be able to infer a subtype from a template literal type. For example:

const canExtend: `myString.${number}` extends `${infer Key}.${number}` ? true : false = true;

πŸ“ƒ Motivating Example

This will allow cross-matching between Values and Paths when traversing an object with dot notation.

πŸ’» Use Cases

See line 35, in this Playground

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