【TypeScript】関数

function

function bmi(height: number, weight: number):number {
  return weight / (height * height)
}

console.log(bmi(1.91, 100))

無名関数

let bmi: (height: number, weight: number) => number = function(height: number, weight: number): number {
  return weight / (height * height)
}

console.log(bmi(1.91, 100))

アロー関数

let bmi: (height: number, weight: number) => number = (
  height: number,
  weight: number
): number =>  weight / (height * height)

console.log(bmi(1.91, 100))

オプショナルなパラメータ

// printableはオプショナル
let bmi = (height: number, weight: number, printable?: boolean): number => {
  const bmi: number = weight / (height * height)
  if (printable) {
    console.log({ bmi })
  }
  return bmi
}
// trueはなくても可
bmi(2.4, 92, true)

デフォルトパラメータ

// rateのデフォルト値は1.1
const nextYearSalary = (currentSalary: number, rate: number = 1.1): number => {
  return currentSalary * rate
}

console.log(nextYearSalary(1000))

残余引数

const reducer = (accumulator: number, currentValue: number): number => {
  console.log({ accumulator, currentValue})
  return accumulator + currentValue
}

const sum: (...values: number[]) => number = (...values: number[]): number => {
  return values.reduce(reducer)
}

console.log(sum(1, 2, 3, 4, 5));

オーバーロード

function double(value: number): number
function double(value: string): string

function double(value: any): any {
  console.log(typeof value)
  if (typeof value === "number") {
    return value * 2 
  } else {
    return value + value
  }
}

console.log(double(200))
// > 400
console.log(double("Foo "))
// > Foo Foo
// console.log(double(true))
// error TS2769: No overload matches this call.
// Overload 1 of 2, '(value: number): number', gave the following error.
//   Argument of type 'boolean' is not assignable to parameter of type 'number'.
// Overload 2 of 2, '(value: string): string', gave the following error.
//   Argument of type 'boolean' is not assignable to parameter of type 'string'.