Generics

跟 C# 差不多，定義方法時不指定型別，使用時再決定

• 一般寫法

function theSame<T>(something: T): T {
  return something
}

// 可以指定預設型別
function theSame2<T = string>(something: T): T {
  return something
}

const result: number = theSame(123)
const result2: string = theSame2('test')

console.log(result) // 123
console.log(result2) // test

• 多個參數

function swap<T, U>(tuple: [T, U]): [U, T] {
  return [tuple[1], tuple[0]]
}

let result = swap([7, 'seven'])
console.log(result) // [ 'seven', 7 ]

• 泛型約束：<T extends something>

// 教職員
interface Faculty {
  id: number
}

interface Teacher extends Faculty {
  subject: string
}

interface Student extends Faculty {
  score: number
}

// 限定要傳入教職員
function theSame<T extends Faculty>(someone: T): T {
  console.log(someone.id) // 只有 id 這個共同屬性能調用
  return someone
}

const student: Student = { id: 1, score: 87 }
console.log(theSame(student))

• 泛型類別

class Something<T> {
  public value: T

  public constructor(value: T) {
    this.value = value
  }
}

const something: Something<number> = new Something<number>(123)
console.log(something.value)

const something2: Something<string> = new Something<string>('Kappa')
console.log(something2.value)