Using TypeScript's features to create a type safe validation flow

demo-with-type-guards.ts

// Define your own interfaces and functions:

interface MyForm { name: string; }

function getFormData(): Unvalidated<MyForm> {
  return { name: 'John' } as Unvalidated<MyForm>;
}

function process(myForm: Validated<MyForm>) { }

function isValid(input: any): input is Validated<MyForm> {
  return input.name != null && input.name.trim().length > 0
}

// usage

const formData = getFormData();
process(formData);   // type error
if (isValid(formData)) {
  process(formData); // ok   
}

demo.ts

// Define your own interfaces and functions:

interface MyForm { name: string; }

function getFormData(): Unvalidated<MyForm> {
  return { name: 'John' } as Unvalidated<MyForm>;
}

function process(myForm: Validated<MyForm>) { }

function validate(input: Unvalidated<MyForm>): Validated<MyForm> {
  if (input.name == null || input.name.trim().length === 0) {
    throw new Error("Invalid Form Data!")
  }
  return input as any as Validated<MyForm>;
}

// usage
const formData = getFormData();
process(formData);  // type error
const validated = validate(formData);
process(validated); // ok

  
// https://www.typescriptlang.org/play/?ssl=38&ssc=2&pln=25&pc=1#code/PQKhCgAIUhhAnApgQwC6IM6WZAdgewFsBLXZAG0lQE8AHRSfAMz2UIYAMAVDyJ-eFQAWDACZocNepA4ARDgDooMAAJTEtZPDaQurdoxaoReIqQpU6iZZDVXN2wpFnCGyAMaoArhfGpJVsrA4OqQAHJmZOQAPFwANM4AfJAAvJAA3pAA+uoAXLqQAL6QAGTOANzg4KAQ0OGRFqHGaJCkosTuaJjCLcYMXriiiPDk1KQA5pB+OMRYBKiQ1IgLAG4UxH6Iokp1KhiIDABq65uiNnsH9SRRQSFWkACquGvkG12isclpEdcU0QDkTxeb3Qon+CS4iUq1TANh+5koTSELTaHS6WGaCz6kAGQxGY1wk2mrSwwNOO1U+wYQJO73OVKuCNuoWOr1On1SjKiANZIK24N0UKq1WAU0QhHwVVI6HgTA8DAAstQAGICJyZMjsfIYVDwCblIpVJgDTzEfC4SDjZaq+CEWQSAAUAEp8jS2e9okqbYRkukoJAkN54BaNWxEPl-gApfBCXD-IrYLBuvkfL1qoWFI0m1Bmi20eD4dyYDAOwgqtX5XnstO2xJOjKG8DG3Cm82QMldB2kWheVCu5600Ge8u1l2QKsems+jL+4gsLu4HuoBSahgpNK4LzkSgAHx3rUXvZXYYUuuIhGdCnIiEJxlS68gAAZ637IG-hAWAO54RDfgCi8AFvADoAEQAJIDmykDes4EgAIQgU6-qZm+gZeMGB5Lom2C4NQ2ETkOU4ZlUwCil4GDIFaVTuOaOp8Gq9r+JyVqoN6jHIM6lSkZA+aFsWDr8La7FOga75iWJ3GhMMQHgDRuB0R2oKcopiACQxEgieAvFFhgJYqaIIniUZ77cfgADWQA)

types.ts

/**
 * Creates a nominal type of name `T` for the data type `D`.
 * @typeparam T name of the nominal type
 * @typeparam D the actual data type
 */
export type Nominal<T, D> = { _type: T } & D;

/**
 * Nominal type that indicates that the underlying data is not yet validated.
 * @see Validated
 * @see Nominal
 */
export type Unvalidated<T> = Nominal<'Unvalidated', T>;

/**
 * Nominal type that indicates that the underlying data is validated.
 * @see Unvalidated
 * @see Nominal
 */
export type Validated<T> = Nominal<'Validated', T>;