I'm trying to convert a generic readonly string[] into an object with known keys. But I encounter the error:

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Record<T[number], number>'.
  No index signature with a parameter of type 'string' was found on type 'Record<T[number], number>'.

Code:

class ArrayToObject<T extends readonly string[]> {
  constructor (private keys : T) {}

  GET_OBJECT () {
    const all = {} as Record<T[number], number>
    for (const index in this.keys) {
      all[this.keys[index]] = index
    }
    return all;
  }
}

What I basically want from the generic is something similar to this:

const a = ["a", "b"] as const
const o : Record<typeof a[number], number> = {
  a : 1,
  b : 2
}

playground

CodePudding user response：

You can get the string union from your readonly string using type inference, like this:

const a = ["a", "b"] as const;

type StringFromReadonlyArray<T extends readonly string[]> = T extends readonly (infer U)[] ? U : string;

const tooMany: Record<StringFromReadonlyArray<typeof a>, number> = {
  a: 1,
  b: 2,
  c: 3, // <-- Error: Type '{ a: number; b: number; c: number; }' is not assignable to type 'Record<"a" | "b", number>'.
};

const notEnough: Record<StringFromReadonlyArray<typeof a>, number> = { // <-- Error: Property 'b' is missing in type '{ a: number; }' but required in type 'Record<"a" | "b", number>'.
  a: 1,
};

const justRight: Record<StringFromReadonlyArray<typeof a>, number> = {
  a: 1,
  b: 2,
};

TypeScript Playground

That StringFromReadonlyArray utility type extracts the string union from your readonly array if possible, otherwise it resolves to the generic string type.

CodePudding user response：

It is because the type system can only know T extends readonly string[] but it cannot infer its elements' type correctly. Defining T as the element type can help the type system to infer it.

Also, the for...in statement will result in a string key value. You need to convert it into number or use Array.keys() or Array.entries() to get the numeric index.

class ArrayToObject<T extends string> {
  constructor (private keys : readonly T[]) {}
  GET_OBJECT () {
    const all = {} as Record<T, number>
    for (const [index, value] of this.keys.entries()) {
      all[value] = index //   1 if you want 1-based
    }
    return all;
  }
}

playground