TypeScript 高级类型：利用模板字面量与类型守卫处理日期字符串

type Person = 'Jeff' | 'Maria'
type Greeting = `hi ${Person}!` // Template literal type
const validGreeting: Greeting = `hi Jeff!` // 
// note that the type of `validGreeting` is the union `"hi Jeff!" | "hi Maria!`
const invalidGreeting: Greeting = `bye Jeff!` // 
// Type '"bye Jeff!"' is not assignable to type '"hi Jeff!" | "hi Maria!"

type Person = 'Jeff' | 'Maria'
type Greeting = `hi ${Person}!`
type LoudGreeting = Uppercase<Greeting> // Capitalization of template literal type
const validGreeting: LoudGreeting = `HI JEFF!` // 
const invalidGreeting: LoudGreeting = `hi jeff!` // 
// Type '"hi Jeff!"' is not assignable to type '"HI JEFF!" | "HI MARIA!"

let age: string | number = getAge();
// `age` is of type `string` | `number`
if (typeof age === 'number') {
  // `age` is narrowed to type `number`
} else {
  // `age` is narrowed to type `string`
}

type Dog = { type: 'dog' };
type Horse = { type: 'horse' };
//  custom type guard, `pet is Dog` is the type predicate
function isDog(pet: Dog | Horse): pet is Dog {
  return pet.type === 'dog';
}
let animal: Dog | Horse = getAnimal();
// `animal` is of type `Dog` | `Horse`
if (isDog(animal)) {
  // `animal` is narrowed to type `Dog`
} else {
  // `animal` is narrowed to type `Horse`
}

type oneToNine = 1|2|3|4|5|6|7|8|9
type zeroToNine = 0|1|2|3|4|5|6|7|8|9
/**
 * Years
 */
type YYYY = `19${zeroToNine}${zeroToNine}` | `20${zeroToNine}${zeroToNine}`
/**
 * Months
 */
type MM = `0${oneToNine}` | `1${0|1|2}`
/**
 * Days
 */
type DD = `${0}${oneToNine}` | `${1|2}${zeroToNine}` | `3${0|1}`
/**
 * YYYYMMDD
 */
type RawDateString = `${YYYY}${MM}${DD}`;
const date: RawDateString = '19990223' // 
const dateInvalid: RawDateString = '19990231' //31st of February is not a valid date, but the template literal doesnt know!
const dateWrong: RawDateString = '19990299'//  Type error, 99 is not a valid day

type Brand<K, T> = K & { __brand: T };
type DateString = Brand<RawDateString, 'DateString'>;
const aDate: DateString = '19990101'; // 
// Type 'string' is not assignable to type 'DateString'

/**
 * Use `moment`, `luxon` or other date library
 */
const isValidDate = (str: string): boolean => {
  // ...
};
//User-defined type guard
function isValidDateString(str: string): str is DateString {
  return str.match(/^\d{4}\d{2}\d{2}$/) !== null && isValidDate(str);
}

/**
 *   Usage in type narrowing
 */
// valid string format, valid date
const date: string = '19990223';
if (isValidDateString(date)) {
  // evaluates to true, `date` is narrowed to type `DateString` 
}
//  valid string format, invalid date (February doenst have 31 days)
const dateWrong: string = '19990231';
if (isValidDateString(dateWrong)) {
  // evaluates to false, `dateWrong` is not a valid date, even if its shape is YYYYMMDD 
}
/**
 *   Usage in factory function
 */
function toDateString(str: RawDateString): DateString {
  if (isValidDateString(str)) return str;
  throw new Error(`Invalid date string: ${str}`);
}
//  valid string format, valid date
const date1 = toDateString('19990211');
// `date1`, is of type `DateString`
//  invalid string format
const date2 = toDateString('asdf');
//  Type error: Argument of type '"asdf"' is not assignable to parameter of type '"19000101" | ...
//  valid string format, invalid date (February doenst have 31 days)
const date3 = toDateString('19990231');
//  Throws Error: Invalid date string: 19990231