import ConstJson from './config.json' as const;

Suggestion

The ability to get const types from a JSON configuration file.

If the json is:

{
  "appLocales": ["FR","BE"]
}

I want to import the JSON and get a {appLocales: "FR" | "BE"} type instead of string

As suggested here, this could rely on ES import attributes (stage 4, ES2025, TypeScript 5.3):

import data from "./data.json" with { type: "json", const: true };

Use Cases

The current approach gives a too broad type string. I understand it makes by default, but having the possibility to import a narrower type would be helpful: it would permit me to avoid maintaining both a runtime locale list + a union type that contains the values that are already in the list, ensuring my type and my runtime values are in sync.

It is possible to type a JSON file explicitly using allowArbitraryExtensions: true and a .d.json.ts file, but this doesn't permit to infer the type from the JSON directly, requires explicit typing and maintenance.

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Links:

This feature has been mentioned:

• by @qm3ster and @maartenth https://github.com/microsoft/TypeScript/pull/29510#issuecomment-456833107
• by @m-b-davis https://github.com/Microsoft/TypeScript/issues/26552#issuecomment-496925984

Search Terms

json const assertion import attributes with allowArbitraryExtensions

Bắt đầu bằng cách xem lại phần thảo luận của issue, tài liệu về import-attributes của TypeScript 5.3 và cách tiếp cận allowArbitraryExtensions hiện có. Công việc được xem là hoàn tất khi một JSON import sử dụng thuộc tính const được đề xuất có thể suy luận các giá trị literal như "FR" | "BE" mà không thay đổi các import hiện có hoặc JavaScript được phát ra.