Validation in TS (version 2)

validation.ts

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

const validate = <T, U = boolean | string>(
  validations: Validation<T, U>[],
  fields: T
): ValidationResult<T, U> =>
  validations
    .map(validation => validation(fields))
    .reduce((acc, a) => Object.assign(acc, a), {});

// Example

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

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

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

type FieldValues = typeof fieldValues;

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

console.log(validate<FieldValues>(validationRules, fieldValues));
// {a: "Needs to be greater than 4", b: true}

console.log(
  validate<FieldValues, boolean | string | (boolean | string)[]>(
    [
      ({ a }) => ({ a: [isGt4(a) ? true : "Needs to be greater than 4!"] }),
      ({ c }) => ({ c: isUserNameDefined(c) })
    ],
    fieldValues
  )
);
// {a: ["Needs to be greater than 4!"], c: "Empty!"}