CODEWITHSUNDEEP
jsontypescriptdynamictypesimport
Slavik Meltser
Slavik Meltser

Reputation: 10371

Import JSON file in TypeScript declaration (*.d.ts files)

I am writing a declaration file which should declare a global type and strict to a specific list of string phrases.
These string phrases are actually part of property keys in an object located in a JSON file.

Two questions:

  1. Is it possible to import a JSON file to a declaration file and process it using basic functions, like Object.keys and Array.map?
  2. Yet, I wonder, if it is even possible to use JS array of strings to define a type in TS?

Let me give you a code example.

Let's say we have the following JSON file named data.json:

{
  "someList": {
    "#key1": {"a": 1, "b": 2},
    "#key2": "some value",
    "#key3": 1234
  }
}

Now, I want to create the following declaration file global.d.ts:

import data from './data.json';

declare global {
  type AllowedKeys = Object(data.someList).map(key => key.substr(1));
}

Basically, I need the type to be defined (dynamically as JSON file changes) like this:

type AllowedKeys = "key1" | "key2" | "key3";

Any help or at least guidance would be appreciated.

Upvotes: 3

Views: 2223

Answers (1)

kouak
kouak

Reputation: 773

EDIT: You can now use template string literals to infer sub-string starting from given character, like so:

import data from "./data.json";
export type AllowedKeys = keyof typeof data["someList"] extends `#${infer K extends string}` ? K : never;
// This is equivalent to
// export type AllowedKeys = "key1" | "key2" | "key3"

You can have typescript infer a type from data in a json-file.

import data from "./data.json";


export type AllowedKeys = keyof typeof data["someList"];
// This is equivalent to
// export type AllowedKeys = "#key1" | "#key2" | "#key3"

The resulting d.ts file looks like this:

import data from "./data.json";
export declare type AllowedKeys = keyof typeof data["someList"];

As far as I know, there's no way to manipulate string literals in typescript (i.e, removing the #).

See those github issues :

Upvotes: 3

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