Validation in TS

validation.ts

type ValidationResult<T, U> = Partial<{ [Key in keyof T]: U }>;
type Validation<T, U> = (input: T) => ValidationResult<T, U>;

const mergeAll = <T, U>(
  results: ValidationResult<T, U>[]
): ValidationResult<T, U> =>
  results.reduce((acc, a) => Object.assign(acc, a), {});

const validate = <T, U = boolean | string>(
  validations: Validation<T, U>[]
): Validation<T, U> => {
  return fields => {
    return mergeAll<T, U>(validations.map(validation => validation(fields)));
  };
};

// Validators
const isGt4 = (input: number): boolean => input > 4;
const isGT8 = (input: number): boolean => input > 8;
const isEmpty = (input: string) => input.length === 0;

// specific validation function
const isUserNameDefined = (input: string) => (isEmpty(input) ? true : "Empty!");

// Values
const fieldValues = {
  a: 3,
  b: 9,
  c: "Test input"
};

type FieldValues = typeof fieldValues;

const validateForm = validate<FieldValues>([
  ({ a }: FieldValues) => ({
    a: isGt4(a) ? true : "Needs to be greater than 4"
  }),
  ({ b }: FieldValues) => ({
    b: isGT8(b) ? true : "Needs to be greater than 8"
  })
]);

const validateOtherForm = validate<
  FieldValues,
  boolean | string | (boolean | string)[]
>([
  ({ a }) => ({ a: [isGt4(a) ? true : "Needs to be greater than 4!"] }),
  ({ c }) => ({ c: isUserNameDefined(c) })
]);

console.log(validateForm(fieldValues));
// {a: "Needs to be greater than 4", b: true}
console.log(validateOtherForm(fieldValues));
// {a: ["Needs to be greater than 4!"], c: "Empty!"}