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

Search Terms

json const assertion import

Suggestion

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

IE if the json is:

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

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

Use Cases

Current approach gives a too broad type string. I understand it may make sense as a 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.

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 mentionned:

• 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

[TheMrZZ]

I’m for option #3. This one looks similar to the current “as const” syntax:

const x = {...} as const

It makes it more intuitive. Definitely a killer feature for config-based code if Typescript adopts it.

[Hawkbat]

This would be extremely useful for adding direct support for JSON schemas to TypeScript. This can technically be accomplished with generators right now, but it would be so much more elegant to be able to use mapped types (for example, https://github.com/wix-incubator/as-typed) to map an imported JSON schema to its corresponding TypeScript type. It isn’t currently possible to use this approach with a JSON import since the type property of each schema object will be a string instead of 'boolean' | 'string' | 'number' | ....