Description
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 Const contexts for literal expressions #29510 (comment)
- by @m-b-davis Treat JSON types more literally #26552 (comment)
Search Terms
json const assertion import attributes with allowArbitraryExtensions