Link object key with return value returned function

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// try trim string
export default function safeTrim(value: unknown): string | undefined {
  if (typeof value === 'string') {
    return value.trim();
  }
  return undefined;
}

// index and items used for array
type Validator<T extends object> = (item: T, index?: number, items?: T[]) => null | { [key in keyof T]: string[] };

export function emptyValidator<T extends object>(keyName: keyof T): Validator<T> {
  return (item) => {
    if (!safeTrim(item[keyName])) {
      // @ts-ignore
      const error: { [key in keyof T]: string[]} = { // line 18
          [keyName]: [`Field should not be empty`],
        };
        return error;
    }
    return null;
  };
}

// ======= examples usage ======== 

const testObject = {
    propThatReturnNull: 'some string',
    propThatReturnError1: '',
    propThatReturnError2: undefined,
    propThatReturnError3: null
} as const;

emptyValidator('propThatReturnNull')(testObject); // should return null
emptyValidator('propThatReturnError1')(testObject); // should return {propThatReturnError1: [`Field should not be empty`]}
emptyValidator('propThatReturnError2')(testObject); // should return {propThatReturnError2: [`Field should not be empty`]}
emptyValidator('propThatReturnError3')(testObject); // should return {propThatReturnError3: [`Field should not be empty`]}

On line 18 i get error Type '{ [x: string]: string[]; }' is not assignable to type '{ [key in keyof T]: string[]; }'.

I expected that keyName type is "keyof T" instead of "string".

How do I mast link object key and returned object key type?

example on ts playgound

typescript
1 Answers

The { [key in keyof T]: string[] } type is a mapped type and it can't be inferred automatically by TypeScript in the function implementation, so you have to type the return value explicitly:

export function emptyValidator<T extends object>(keyName: keyof T): Validator<T> {
  return (item) => {
    if (!safeTrim(item[keyName])) {
      return {
          [keyName]: [`Field should not be empty`],
        } as { [key in keyof T]: string[] };
    }
    return null;
  };
}
user_37381710
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