simple-function-validator.ts

type Validator<Input, Error> = (input: Input) => Error[];

type ExtractValidatorError<V extends Validator<any, any>> = V extends Validator<any, infer A> ? A : never;
function error<Error>(error: Error): Error[] {
  return [error];
}
function ok(): any[] {
  return [];
}

function tryCatch<Input, Error>(fn: (input: Input) => void, onError: (e: unknown) => Error): Validator<Input, Error> {
  return input => {
    try {
      fn(input);
      return ok();
    } catch (e) {
      return error(onError(e));
    }
  }
}

function validations<Input>(): <Validators extends Array<Validator<Input, any>>>(...validations: Validators) => Validator<Input, ExtractValidatorError<Validators[number]>> {
  return function consumeValidators<Validators extends Array<Validator<Input, any>>>(...validations: Validators) {
    type Error = ExtractValidatorError<Validators[number]>;
    return composeValidations<Input, Error>(...validations);
  }
}

function composeValidations<Input, Error>(...args: Validator<Input, Error>[]): Validator<Input, Error> {
  return input => {
    const errors: Error[] = [];

    for (const validator of args) {
      errors.push(...validator(input));
    }
    return errors;
  }
}

////////////////// USAGE //////////////////

const user = {
  /** a name */
  name: "Gal Schlezinger",
  /** an age */
  age: 29,
  /** an occupation */
  occupation: "Developer",

  type: "human" as const,
};

type User = typeof user;

function mustBe18(user: Pick<User, 'age'>): string[] {
  if (user.age < 18) {
    return error(`User age must be above 18. Got: ${user.age}`);
  }
  return ok();
}

function goatOrHuman(user: { type: "human" } | { type: "goat" }): string[] {
  return [];
}

const nameMustBePreset = tryCatch((input: Pick<User, 'name'>) => {
  if (input.name.trim().length === 0) {
    throw new Error("Name must be present");
  }
}, e => (e as Error).message)

const validator = validations<User>()(mustBe18, nameMustBePreset, goatOrHuman);

function validate(input: Parameters<typeof validator>[0]) {
  console.log({ input, output: validator(input) });
}

validate(user);
validate({
  age: 17,
  name: "",
  occupation: "",
  type: "human",
})