009--函数(基本实例和函数类型)

基本函数

function  add(x, y) {
  return x + y
}

let myAdd = function(x, y) {
  return x + y
}

let z = 100
function addToZ(x, y) {
  return x + y + z
}

 给刚才的函数添加完整的函数类型

//给刚才的函数添加类型
function  add(x: number, y: number): number {
  return x + y
}
//给函数添加完整的函数类型
let myAdd:(baseValue: number, increment: number)=> number = function(x: number, y: number): number {
  return x + y
}

函数类型推断

//给函数添加完整的函数类型
let myAdd:(baseValue: number, increment: number)=> number = function(x: number, y: number): number {
  return x + y
}
//上述函数可以拆分成下面两种
// let myAdd = function(x: number, y: number): number {
//   return x + y
// }
// let myAdd:(baseValue: number, increment: number)=> number = function(x, y) {
//   return x + y
// }
//都会根据类型推断来推断出参数类型和返回值类型

可选参数

//在TypeScript中的参数只要定义了就是必须的,js中默认都是可选的
function bulidName(firstName: string, lastName: string): string {
  return firstName + '' + lastName 
}

// let result1 = bulidName('Bob')//报错,只传了一个参数
// let result2 = bulidName('Bob', 'Jack', 'Lisa')//报错,传入了三个参数
let result3 = bulidName('Bob', 'Jack')

//改造一下,让第二个参数可选
function bulidName(firstName: string, lastName?: string): string {
  if(lastName) {
  return firstName + '' + lastName 
  }else {
    return firstName
  }
}
let result = bulidName('Bob')
//可选参数,必须跟在必须参数的后面 

函数默认值

//函数默认值
function bulidName(firstName: string, lastName = 'Smith'): string {
  return firstName + '' + lastName 
}

let result = bulidName('Bob')
console.log(result)//BobSmith

//函数默认值
function bulidName(firstName = 'Bob', lastName: string ): string {
  return firstName + '' + lastName 
}

let result = bulidName('Jack', 'smith')
let result1 = bulidName(undefined, 'Jons')
console.log(result)//Jacksmith
console.log(result1)//BobJons
//如果想把默认参数放在前面,使用函数默认值的值的话则在函数调用时一定要在相应位置传入undefined

剩余参数

function bulidName(firstName = 'Bob', ...restOfName: string[] ): string {
  return firstName + '' + restOfName
}

let result = bulidName('Jack', 'smith')
let result1 = bulidName(undefined, 'Jons')
let result3 = bulidName('Jack', 'smith', 'Niclos')
console.log(result)//Jacksmith
console.log(result1)//BobJons
console.log(result3)//Jacksmith,Niclos
//如果想把默认参数放在前面,使用函数默认值的值的话则在函数调用时一定要在相应位置传入undefined

剩余参数编译后

function bulidName(firstName) {
    if (firstName === void 0) { firstName = 'Bob'; }
    var restOfName = [];
    for (var _i = 1; _i < arguments.length; _i++) {
        restOfName[_i - 1] = arguments[_i];
    }
    return firstName + '' + restOfName;
}
var result = bulidName('Jack', 'smith');
var result1 = bulidName(undefined, 'Jons');
var result3 = bulidName('Jack', 'smith', 'Niclos');
console.log(result); //Jacksmith
console.log(result1); //BobJons
console.log(result3);

带有剩余参数的函数定义

function bulidName(firstName = 'Bob', ...restOfName: string[] ): string {
  return firstName + '' + restOfName
}
//带有剩余参数的函数定义
let bulidNameFn: (fname: string, ...rest: string[]) => string = bulidName

2019-05-28  11:38:16