An example of a helper function to map an object with ISO6801 date string properties to an object with proper Date properties

json-datestring-to-date.ts

// v Scroll down to see the example first v
// v--------------------------------------v

// a mapped type that is a union of the names of properties of T that are of type string
type OnlyStringProperties<T> = { [Key in keyof T]: T[Key] extends string ? Key : never }[keyof T];

// From an original object of type T, return another object where the properties
// whose name is passed are parsed to Date 
function convertStringPropertiesToDates<T, K extends OnlyStringProperties<T>>(
  original: T,
  ...propNames: K[]
): T & Record<K, Date> {
  const extractedDates: Partial<Record<K, Date>> = {};
  for (const propName of propNames) {
    const dateString = (original[propName] as unknown) as string;
    extractedDates[propName] = new Date(dateString);
  }

  const populatedDates = extractedDates as Record<K, Date>;
  return { ...original, ...populatedDates };
}

// A little example
// ================

// say we have a type that represents some JSON paylod we are getting.
// JSON doesn't have dates, so all we get are ISO 6801 date strings ...

interface IncomingJsonType {
  readonly counter: number;
  readonly someDate1: string;
  readonly someDate2: string;
  readonly things: string[];
}

const json: IncomingJsonType = {
  counter: 4,
  someDate1: '2019-06-14T20:32:01.071Z',
  someDate2: '2019-05-14T20:32:01.071Z',
  things: ['a', 'b']
};

// ... but in the code, we mostly want that, but with real dates :
interface BusinessObjectWithRealDates {
  readonly counter: number;
  readonly someDate1: Date;
  readonly someDate2: Date;
  readonly things: string[];
}

// we could parse the date strings one by one  (with `new Date(ISOString)`) ....
// but we'd have to do it every time this happens

// ... or we can use some TypeScript tricks such as `Mapped Types`
// here, TypeScript only allows us to pass as params the names of properties
// from our object that are `string`s
const result = convertStringPropertiesToDates(json, 'someDate1', 'someDate2');


// Here typescript does not complain ! 
// the result type is compatible with the interface we wanted
const businessObj: BusinessObjectWithRealDates = result;


// More ...
// ========
// Try it in the TypeScript playground to see the inferred types and compilation errors from TypeScript ! https://www.typescriptlang.org/play/

// Inspired by :
// - https://medium.com/dailyjs/typescript-create-a-condition-based-subset-types-9d902cea5b8c
// - https://stackoverflow.com/questions/50900533/how-can-i-extract-the-names-of-all-fields-of-a-specific-type-from-an-interface-i/50900933
// 
// More info about advanced types in TypeScript documentation : 
// - https://www.typescriptlang.org/docs/handbook/advanced-types.html
// - https://www.typescriptlang.org/docs/handbook/utility-types.html
//