How to do Typesafe JSON with fields of Enum type?

Question

I have some JSON files that conform to TypeScript interfaces that I have defined.

For the most part, when importing such JSON files and assigning them to typed variables, TypeScript is able to infer the type signature automatically (see behaves exactly as I want in the code below). However when the type includes string enums, it no longer works (see DOES NOT behave).

The following is a working Minimal, Reproducible Example:

valid.json

{ "id": 3.14159 }

invalid.json

{ "id": "3.14159" }

validEnum.json

{ "color": "red" }

invalidEnum.json

{ "color": "chartreuse" }

index.ts

import validJson from './valid.json'
import invalidJson from './invalid.json'
import validJsonEnum from './validEnum.json'
import invalidJsonEnum from './invalidEnum.json'

type ColorType = 'red' | 'blue' | 'yellow'

type IJsonType = {"id": number}
type IJsonTypeWithEnum = {"color": ColorType}

// behaves exactly as I want
const a: IJsonType = validJson    // no error
const b: IJsonType = invalidJson  // ERROR: Type 'string' is not assignable to type 'number'.

// DOES NOT behave as I want: SHOULD NOT error
const c: IJsonTypeWithEnum = validJsonEnum    // ERROR: Type 'string' is not assignable to type 'ColorType'.

// DOES NOT behave as I want: error should be that "chartreuse" is not assignable to type 'ColorType'
const d: IJsonTypeWithEnum = invalidJsonEnum  // ERROR: Type 'string' is not assignable to type 'ColorType'.

I can make the error go away with type IJsonTypeWithEnum = {"color": string}, but that defeats the purpose.

Is there any work-around or a compiler switch to make TypeScript recognize enum values inside JSON as strings? Or is it a dead-end TypeScript limitation for JSON type inference?

Answer 1

Try to use type assertion

const data: IJsonType = jsonData as IJsonType;