TypeScript 类型问题

Question

现有一个函数接收a,b,c三个参数，a,b都是选传，但如果传了a,c需要必传，那么应该如何写类型呢

Answer 1

函数重载咯

function test(a: string, b: string, c: string): string;
function test(): string;
function test() {
    return 'test';
}

Answer 2

类型使用函数重载。

如果传了a,c需要必传
这是函数体里的判断，和类型无关。

按我理解，其实就是没有参数、一个参数、两个参数、三个参数的不同场景...

函数重载的方式很多：

// type定义
type Tfn = (a: number, b: number) => number;
let fn1: Tfn
fn1 = function(a, b) {
  return  a + b
}
fn1(1, 2)


// type定义重载
type Tfn = {
  (a: string): string;
  (a: number, b: number): number;
}
let fn1: Tfn
fn1 = function(a: any, b?: any): any {
  if (b) {
    return a + b
  } else {
    return a
  }
}
fn1('1')
//  ^ = let fn1: (a: string) => string (+1 overload)
fn1(1, 2)
//  ^ = let fn1: (a: number, b: number) => number (+1 overload)
fn1(1)  //Error



// interface定义
interface Ifn {
  (x: string): string;
}
let fn1: Ifn
fn1 = function(a) {
  return a
}
fn1('1')

// interface定义重载
interface Ifn {
  (x: string): string;
  (x: number, y: number): number;
}
let fn1: Ifn
fn1 = function(a: any, b?: any): any {
  if (b) {
    return a + b
  } else {
    return a
  }
}
fn1('1')
//  ^ = let fn1: (a: string) => string (+1 overload)
fn1(1, 2)
//  ^ = let fn1: (a: number, b: number) => number (+1 overload)
fn1(1)  //Error



// let & typeof 声明
let fn: {
  (x: string): string;
}
let fn1: typeof fn
fn1 = function(a) {
  return a
}
fn1('1')

// let & typeof声明重载
let fn: {
  (x: string): string;
  (x: number, y: number): number;
}
let fn1: typeof fn
fn1 = function(a: any, b?: any) {
  if (b) {
    return a + b
  } else {
    return a
  }
}
fn1('1')
//  ^ = let fn1: (a: string) => string (+1 overload)
fn1(1, 2)
//  ^ = let fn1: (a: number, b: number) => number (+1 overload)
fn1(1)  //Error



// function声明
function fn(x: string): string {
  return x
}
fn('1')

// function声明重载：fn实现必须紧跟着fn头声明
function fn(x: string): string;
function fn(x: number, y: number): number;
function fn(x: any, y?: any) {
  if (y) {
    return x + y
  } else {
    return x
  }
}
fn('1')
//  ^ = let fn: (a: string) => string (+1 overload)
fn(1, 2)
//  ^ = let fn: (a: number, b: number) => number (+1 overload)
fn(1)  //Error