"Dev vs production type checking" rewrite in TypeScript

type-checking-rewrite-in-typescript.ts

// Based on: https://gist.github.com/ericelliott/e2dd9cbe5c64e44d6122ba76ec7dbf55#file-dev-vs-production-type-checking-js

type Timestamp = number // Date.now() returns number, so type alias gives better documentation

// good type names provide good documentation
type Employee = {
  name: string,
  hireDate?: Timestamp,
  title?: string,
}

const createEmployee = (e: Employee): Employee => ({
  name: e.name,
  hireDate: e.hireDate ?? Date.now(),
  title: e.title ?? 'Worker Drone',
})

// if the default values could be undefined (TS always errors when there is unchecked access to optional property),
// this function can be even simpler
const ce = (e: Employee): Employee => e

// if there are multiple functions with the same type signature, we can make a type for them
type EmployeeCreator = (e: Employee) => Employee

const c: EmployeeCreator = e => e

// In development, a framework doesn't have to wrap calls:
const emp1 = createEmployee({ name: 'foo' }) // works

const emp2 = createEmployee() // fails
//           ^^^^^^^^^^^^^^^^
// Expected 1 arguments, but got 0. ts(2554)

const emp3 = createEmployee({ name: 1 }) // fails, but would pass in the js example
//                            ^^^^
// Type 'number' is not assignable to type 'string'. ts(2322)

// Call the function directly everywhere
const emp4 = createEmployee({ name: 'foo' })