How to roop typescript's type for params?

January 23, 2024

If I set listData[index][field] = newValue as any, it’s working, but I don’t want to use any.

I still try to use newValue: string | number | Date still shows the error

How do I let the typescript working for the setting listData[index][field] = newValue ?

interface Sons {
  name: string;
  address?: string;
  date?: Date;
  age?: number;
}

interface Parents extends Sons {
  height: string;
}

const parentList: Parents = {
  name: '',
  height: '',
}

const listData: Parents[] = [parentList]


class Test {
  update<K extends keyof Parents>(index: number, field: K, newValue: string ) {
    // Type 'string' is not assignable to type 'Parents[K]'.
    // Type 'string' is not assignable to type 'never'
    listData[index][field] = newValue
}

>Solution :

You can use Parents[K] instead of string for newValue. This is called an Indexed Access type which dynamically looks up the type of a given property key (K) on the Parents interface.

interface Sons {
  name: string;
  address?: string;
  date?: Date;
  age?: number;
}

interface Parents extends Sons {
  height: string;
}

const parentList: Parents = {
  name: '',
  height: '',
}

const listData: Parents[] = [parentList]


class Test {
  update<K extends keyof Parents>(index: number, field: K, newValue: Parents[K] ) {
    listData[index][field] = newValue;
  }
}

declare const test: Test;

test.update(0, "height", "very tall");
test.update(0, "date", new Date());

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