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

首先查看 issue 讨论、TypeScript 5.3 的 import-attributes 文档，以及现有的 allowArbitraryExtensions 方法。完成的标准是：使用提议的 const 属性的 JSON import 可以推断出诸如 "FR" | "BE" 这样的字面量值，同时不改变现有的 import 或生成的 JavaScript。