reference/docs/javascript.md

JavaScript 备忘清单

包含最重要概念、函数、方法等的 JavaScript 备忘单。 初学者的完整快速参考。

入门

介绍

JavaScript 是一种轻量级的解释型编程语言。

• JSON 备忘清单
• JavaScript 中的正则表达式

打印调试

// => Hello world!
console.log('Hello world!');
// => Hello QuickReference
console.warn('hello %s', 'QuickReference');
// 将错误消息打印到 stderr
console.error(new Error('Oops!'));

数字

let amount = 6;
let price = 4.99;

变量

let x = null;
let name = "Tammy";
const found = false;
// => Tammy, false, null
console.log(name, found, x);
var a;
console.log(a); // => undefined

字符串

let single = 'Wheres my bandit hat?';
let double = "Wheres my bandit hat?";
// => 21
console.log(single.length);

算术运算符

5 + 5 = 10     // 加法 Addition
10 - 5 = 5     // 加法 Subtraction
5 * 10 = 50    // 乘法 Multiplication
10 / 5 = 2     // 除法 Division
10 % 5 = 0     // 取模 Modulo

注释

// 此行将表示注释
/* 
多行配置
部署前必须更改
以下配置。
*/

赋值运算符

let number = 100;
// 两个语句都会加 10
number = number + 10;
number += 10;
console.log(number); 
// => 120

字符串插值

let age = 7;
// 字符串拼接
'Tommy is ' + age + ' years old.';
// 字符串插值
`Tommy is ${age} years old.`;

let 关键字

let count; 
console.log(count); // => undefined
count = 10;
console.log(count); // => 10

const 关键字

const numberOfColumns = 4;
// TypeError: Assignment to constant...
numberOfColumns = 8;

JavaScript 条件

if Statement (if 语句)

const isMailSent = true;

if (isMailSent) {
  console.log('Mail sent to recipient');
}

Ternary Operator (三元运算符)

var x=1;

// => true
result = (x == 1) ? true : false;

Operators

true || false;       // true
10 > 5 || 10 > 20;   // true
false || false;      // false
10 > 100 || 10 > 20; // false

逻辑运算符 &&

true && true;        // true
1 > 2 && 2 > 1;      // false
true && false;       // false
4 === 4 && 3 > 1;    // true

比较运算符

1 > 3                 // false
3 > 1                 // true
250 >= 250            // true
1 === 1               // true
1 === 2               // false
1 === '1'             // false

逻辑运算符 !

let lateToWork = true;
let oppositeValue = !lateToWork;
// => false
console.log(oppositeValue); 

空值合并运算符 ??

null ?? 'I win';         //  'I win'
undefined ?? 'Me too';   //  'Me too'
false ?? 'I lose'        //  false
0 ?? 'I lose again'      //  0
'' ?? 'Damn it'          //  ''

else if

const size = 10;

if (size > 100) {
  console.log('Big');
} else if (size > 20) {
  console.log('Medium');
} else if (size > 4) {
  console.log('Small');
} else {
  console.log('Tiny');
}
// Print: Small

switch 语句

const food = 'salad';

switch (food) {
  case 'oyster':
    console.log('海的味道');
    break;
  case 'pizza':
    console.log('美味的馅饼');
    break;
  default:
    console.log('请您用餐');
}

== vs ===

0 == false   // true
0 === false  // false, 不同类型
1 == "1"     // true,  自动类型转换
1 === "1"    // false, 不同类型
null == undefined  // true
null === undefined // false
'0' == false       // true
'0' === false      // false

== 只检查值，=== 检查值和类型。

JavaScript Functions 函数

函数

// 定义函数：
function sum(num1, num2) {
  return num1 + num2;
}
// 调用函数：
sum(3, 6); // 9

匿名函数

// Named function
function rocketToMars() {
  return 'BOOM!';
}
// Anonymous function
const rocketToMars = function() {
  return 'BOOM!';
}

箭头函数 (ES6)

有两个参数

const sum = (param1, param2) => { 
  return param1 + param2; 
}; 
console.log(sum(2,5)); // => 7 

没有参数

const printHello = () => { 
  console.log('hello'); 
}; 
printHello(); // => hello

只有一个参数

const checkWeight = weight => { 
  console.log(`Weight : ${weight}`); 
}; 
checkWeight(25); // => Weight : 25 

简洁箭头函数

const multiply = (a, b) => a * b; 
// => 60 
console.log(multiply(2, 30)); 

从 ES2015 开始提供箭头函数

返回关键字

// 有 return
function sum(num1, num2) {
  return num1 + num2;
}
// 该函数不输出总和
function sum(num1, num2) {
  num1 + num2;
}

调用函数

// 定义函数
function sum(num1, num2) {
  return num1 + num2;
}
// 调用函数
sum(2, 4); // 6

函数表达式

const dog = function() {
  return 'Woof!';
}

函数参数

// 参数是 name
function sayHello(name) {
  return `Hello, ${name}!`;
}

函数声明

function add(num1, num2) {
  return num1 + num2;
}

JavaScript 范围

范围

function myFunction() {
  
  var pizzaName = "Margarita";
  // 这里的代码可以使用 PizzaName
  
}
// 这里的代码不能使用 PizzaName

Block Scoped Variables

const isLoggedIn = true;
if (isLoggedIn == true) {
  const statusMessage = 'Logged in.';
}
// Uncaught ReferenceError...
// 未捕获的引用错误...
console.log(statusMessage);

全局变量

// 全局声明的变量
const color = 'blue';
function printColor() {
  console.log(color);
}
printColor(); // => blue

let vs var

for (let i = 0; i < 3; i++) {
  // 这是“let”的最大范围
  // i 可以访问 ✔️
}
// i 不能访问 ❌

---

for (var i = 0; i < 3; i++) {
  // i 可以访问 ✔️
}
// i 可以访问 ✔️

var 的范围是最近的函数块，而 let 的范围是最近的封闭块。

带闭包的循环

// 打印三次，不是我们的意思。
for (var i = 0; i < 3; i++) {
  setTimeout(_ => console.log(i), 10);
}

---

// 按预期打印 0、1 和 2。
for (let j = 0; j < 3; j++) { 
  setTimeout(_ => console.log(j), 10);
}

变量使用 let 有自己的副本，变量有使用 var 的共享副本。

JavaScript Arrays

数组

const fruits = ["apple", "orange", "banana"];
// 不同的数据类型
const data = [1, 'chicken', false];

属性 .length

const numbers = [1, 2, 3, 4];
numbers.length // 4

索引

// 访问数组元素
const myArray = [100, 200, 300];
console.log(myArray[0]); // 100
console.log(myArray[1]); // 200

可变图表

	add	remove	start	end
push	✅			✅
pop		✅		✅
unshift	✅		✅
shift		✅	✅

方法 .push()

// 添加单个元素：
const cart = ['apple', 'orange'];
cart.push('pear'); 
// 添加多个元素：
const numbers = [1, 2];
numbers.push(3, 4, 5);

将项目添加到末尾并返回新的数组长度。

方法 .pop()

const fruits = ["apple", "orange", "banana"];
const fruit = fruits.pop(); // 'banana'
console.log(fruits); // ["apple", "orange"]

从末尾删除一个项目并返回已删除的项目。

方法 .shift()

let cats = ['Bob', 'Willy', 'Mini'];
cats.shift(); // ['Willy', 'Mini']

从头删除一个项目并返回已删除的项目。

方法 .unshift()

let cats = ['Bob'];
// => ['Willy', 'Bob']
cats.unshift('Willy');
// => ['Puff', 'George', 'Willy', 'Bob']
cats.unshift('Puff', 'George');

将项目添加到开头并返回新的数组长度。

方法 .concat()

const numbers = [3, 2, 1]
const newFirstNumber = 4
    
// => [ 4, 3, 2, 1 ]
[newFirstNumber].concat(numbers)
    
// => [ 3, 2, 1, 4 ]
numbers.concat(newFirstNumber)

如果你想避免改变你的原始数组，你可以使用 concat。

JavaScript 循环

While 循环

while (condition) {
  // code block to be executed
}
let i = 0;
while (i < 5) {        
  console.log(i);
  i++;
}

反向循环

const fruits = ["apple", "orange", "banana"];
for (let i = fruits.length - 1; i >= 0; i--) {
  console.log(`${i}. ${fruits[i]}`);
}
// => 2. banana
// => 1. orange
// => 0. apple

Do...While 语句

x = 0
i = 0
do {
  x = x + i;
  console.log(x)
  i++;
} while (i < 5);
// => 0 1 3 6 10

For 循环

for (let i = 0; i < 4; i += 1) {
  console.log(i);
};
// => 0, 1, 2, 3

遍历数组

for (let i = 0; i < array.length; i++){
  console.log(array[i]);
}
// => 数组中的每一项

Break

for (let i = 0; i < 99; i += 1) {
  if (i > 5) {
     break;
  }
  console.log(i)
}
// => 0 1 2 3 4 5

Continue

for (i = 0; i < 10; i++) {
  if (i === 3) { continue; }
  text += "The number is " + i + "<br>";
}

嵌套循环

for (let i = 0; i < 2; i += 1) {
  for (let j = 0; j < 3; j += 1) {
    console.log(`${i}-${j}`);
  }
}

for...in 循环

const fruits = ["apple", "orange", "banana"];
for (let index in fruits) {
  console.log(index);
}
// => 0
// => 1
// => 2

for...of 循环

const fruits = ["apple", "orange", "banana"];
for (let fruit of fruits) {
  console.log(fruit);
}
// => apple
// => orange
// => banana

JavaScript 迭代器(Iterators)

分配给变量的函数

let plusFive = (number) => {
  return number + 5;  
};
// f 被赋值为 plusFive
let f = plusFive;
plusFive(3); // 8
// 由于 f 具有函数值，因此可以调用它。
f(9); // 14

回调函数

const isEven = (n) => {
  return n % 2 == 0;
}
let printMsg = (evenFunc, num) => {
  const isNumEven = evenFunc(num);
  console.log(`${num} is an even number: ${isNumEven}.`)
}
// Pass in isEven as the callback function
printMsg(isEven, 4); 
// => The number 4 is an even number: True.

数组方法 .reduce()

const numbers = [1, 2, 3, 4];
const sum = numbers.reduce((accumulator, curVal) => {  
  return accumulator + curVal;
});
console.log(sum); // 10

数组方法 .map()

const members = ["Taylor", "Donald", "Don", "Natasha", "Bobby"];
const announcements = members.map((member) => {
  return member + " joined the contest.";
});
console.log(announcements);

数组方法 .forEach()

const numbers = [28, 77, 45, 99, 27];
numbers.forEach(number => {  
  console.log(number);
}); 

数组方法 .filter()

const randomNumbers = [4, 11, 42, 14, 39];
const filteredArray = randomNumbers.filter(n => {  
  return n > 5;
});

JavaScript 对象(Objects)

访问属性

const apple = { 
  color: 'Green',
  price: { bulk: '$3/kg', smallQty: '$4/kg' }
};
console.log(apple.color);      // => Green
console.log(apple.price.bulk); // => $3/kg

命名属性

// 无效键名示例
const trainSchedule = {
  // 由于单词之间的空格而无效。
  platform num: 10, 
  // 表达式不能是键。
  40 - 10 + 2: 30,
  // 除非用引号括起来，否则 + 号无效。
  +compartment: 'C'
}

不存在的属性

const classElection = {
  date: 'January 12'
};
console.log(classElection.place); // undefined

可变的

const student = {
  name: 'Sheldon',
  score: 100,
  grade: 'A',
}
console.log(student)
// { name: 'Sheldon', score: 100, grade: 'A' }
delete student.score
student.grade = 'F'
console.log(student)
// { name: 'Sheldon', grade: 'F' }
student = {}
// TypeError: TypeError：分配给常量变量。

赋值简写语法

const person = {
  name: 'Tom',
  age: '22',
};
const {name, age} = person;
console.log(name); // 'Tom'
console.log(age);  // '22'

删除运算符

const person = {
  firstName: "Matilda",
  age: 27,
  hobby: "knitting",
  goal: "learning JavaScript"
};
delete person.hobby; // or delete person[hobby];
console.log(person);
/*
{
  firstName: "Matilda"
  age: 27
  goal: "learning JavaScript"
}
*/

对象作为参数

const origNum = 8;
const origObj = {color: 'blue'};
const changeItUp = (num, obj) => {
  num = 7;
  obj.color = 'red';
};
changeItUp(origNum, origObj);
// 将输出 8，因为整数是按值传递的。
console.log(origNum);
// 由于传递了对象，将输出“red”
// 通过引用，因此是可变的。
console.log(origObj.color);

速记对象创建

const activity = 'Surfing';
const beach = { activity };
console.log(beach); // { activity: 'Surfing' }

this 关键字

const cat = {
  name: 'Pipey',
  age: 8,
  whatName() {
    return this.name  
  }
};
console.log(cat.whatName()); // => Pipey

工厂函数

// 一个接受 'name'，'age' 和 'breed' 的工厂函数，
//  参数返回一个自定义的 dog 对象。
const dogFactory = (name, age, breed) => {
  return {
    name: name,
    age: age,
    breed: breed,
    bark() {
      console.log('Woof!');  
    }
  };
};

方法

const engine = {
  // 方法简写，有一个参数
  start(adverb) {
    console.log(`The engine starts up ${adverb}...`);
  },  
  // 不带参数的匿名箭头函数表达式
  sputter: () => {
    console.log('The engine sputters...');
  },
};
engine.start('noisily');
engine.sputter();

Getters 和 setters

const myCat = {
  _name: 'Dottie',
  get name() {
    return this._name;  
  },
  set name(newName) {
    this._name = newName;  
  }
};
// 引用调用 getter
console.log(myCat.name);
// 赋值调用 setter
myCat.name = 'Yankee';

JavaScript Classes

Static Methods

class Dog {
  constructor(name) {
    this._name = name;  
  }
  
  introduce() { 
    console.log('This is ' + this._name + ' !');  
  }
  
  // A static method
  static bark() {
    console.log('Woof!');  
  }
}
const myDog = new Dog('Buster');
myDog.introduce();
// Calling the static method
Dog.bark();

Class

class Song {
  constructor() {
    this.title;
    this.author;
  }
  
  play() {
    console.log('Song playing!');
  }
}
const mySong = new Song();
mySong.play();

Class Constructor

class Song {
  constructor(title, artist) {
    this.title = title;
    this.artist = artist;
  }
}
const mySong = new Song('Bohemian Rhapsody', 'Queen');
console.log(mySong.title);

Class Methods

class Song {
  play() {
    console.log('Playing!');
  }
  
  stop() {
    console.log('Stopping!');
  }
}

extends

// Parent class
class Media {
  constructor(info) {
    this.publishDate = info.publishDate;
    this.name = info.name;
  }
}
// Child class
class Song extends Media {
  constructor(songData) {
    super(songData);
    this.artist = songData.artist;
  }
}
const mySong = new Song({ 
  artist: 'Queen', 
  name: 'Bohemian Rhapsody', 
  publishDate: 1975
});

JavaScript Modules

Export

// myMath.js
// Default export
export default function add(x,y){
    return x + y
}
// Normal export
export function subtract(x,y){
    return x - y
}
// Multiple exports
function multiply(x,y){
    return x * y
}
function duplicate(x){
    return x * 2
}
export {
    multiply,
    duplicate
}

Import

// main.js
import add, { subtract, multiply, duplicate } from './myMath.js';
console.log(add(6, 2)); // 8 
console.log(subtract(6, 2)) // 4
console.log(multiply(6, 2)); // 12
console.log(duplicate(5)) // 10
// index.html
<script type="module" src="main.js"></script>

Export Module

// myMath.js
function add(x,y){
    return x + y
}
function subtract(x,y){
    return x - y
}
function multiply(x,y){
    return x * y
}
function duplicate(x){
    return x * 2
}
// Multiple exports in node.js
module.exports = {
    add,
    subtract,
    multiply,
    duplicate
}

Require Module

// main.js
const myMath = require('./myMath.js')
console.log(myMath.add(6, 2)); // 8 
console.log(myMath.subtract(6, 2)) // 4
console.log(myMath.multiply(6, 2)); // 12
console.log(myMath.duplicate(5)) // 10

JavaScript Promises

Promise states

const promise = new Promise((resolve, reject) => {
  const res = true;
  // An asynchronous operation.
  if (res) {
    resolve('Resolved!');
  }
  else {
    reject(Error('Error'));
  }
});
promise.then((res) => console.log(res), (err) => console.error(err));

Executor function

const executorFn = (resolve, reject) => {
  resolve('Resolved!');
};
const promise = new Promise(executorFn);

setTimeout()

const loginAlert = () =>{
  console.log('Login');
};
setTimeout(loginAlert, 6000);

.then() method

const promise = new Promise((resolve, reject) => {    
  setTimeout(() => {
    resolve('Result');
  }, 200);
});
promise.then((res) => {
  console.log(res);
}, (err) => {
  console.error(err);
});

.catch() method

const promise = new Promise((resolve, reject) => {  
  setTimeout(() => {
    reject(Error('Promise Rejected Unconditionally.'));
  }, 1000);
});
promise.then((res) => {
  console.log(value);
});
promise.catch((err) => {
  console.error(err);
});

Promise.all()

const promise1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(3);
  }, 300);
});
const promise2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(2);
  }, 200);
});
Promise.all([promise1, promise2]).then((res) => {
  console.log(res[0]);
  console.log(res[1]);
});

Avoiding nested Promise and .then()

const promise = new Promise((resolve, reject) => {  
  setTimeout(() => {
    resolve('*');
  }, 1000);
});
const twoStars = (star) => {  
  return (star + star);
};
const oneDot = (star) => {  
  return (star + '.');
};
const print = (val) => {
  console.log(val);
};
// Chaining them all together
promise.then(twoStars).then(oneDot).then(print);

Creating

const executorFn = (resolve, reject) => {
  console.log('The executor function of the promise!');
};
const promise = new Promise(executorFn);

Chaining multiple .then()

const promise = new Promise(resolve => setTimeout(() => resolve('dAlan'), 100));
promise.then(res => {
  return res === 'Alan' ? Promise.resolve('Hey Alan!') : Promise.reject('Who are you?')
}).then((res) => {
  console.log(res)
}, (err) => {
  console.error(err)
});

JavaScript Async-Await

Asynchronous

function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}
const msg = async function() { //Async Function Expression
  const msg = await helloWorld();
  console.log('Message:', msg);
}
const msg1 = async () => { //Async Arrow Function
  const msg = await helloWorld();
  console.log('Message:', msg);
}
msg(); // Message: Hello World! <-- after 2 seconds
msg1(); // Message: Hello World! <-- after 2 seconds

Resolving Promises

let pro1 = Promise.resolve(5);
let pro2 = 44;
let pro3 = new Promise(function(resolve, reject) {
  setTimeout(resolve, 100, 'foo');
});
Promise.all([pro1, pro2, pro3]).then(function(values) {
  console.log(values);
});
// expected => Array [5, 44, "foo"]

Async Await Promises

function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}
async function msg() {
  const msg = await helloWorld();
  console.log('Message:', msg);
}
msg(); // Message: Hello World! <-- after 2 seconds

Error Handling

let json = '{ "age": 30 }'; // incomplete data
try {
  let user = JSON.parse(json); // <-- no errors
  console.log( user.name ); // no name!
} catch (e) {
  console.error( "Invalid JSON data!" );
}

Aysnc await operator

function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}
async function msg() {
  const msg = await helloWorld();
  console.log('Message:', msg);
}
msg(); // Message: Hello World! <-- after 2 seconds

JavaScript 请求

JSON

const jsonObj = {
  "name": "Rick",
  "id": "11A",
  "level": 4  
};

另见：JSON 备忘单

XMLHttpRequest

const xhr = new XMLHttpRequest();
xhr.open('GET', 'mysite.com/getjson');

XMLHttpRequest 是一个浏览器级别的 API，它使客户端能够通过 JavaScript 编写数据传输脚本，而不是 JavaScript 语言的一部分。

GET

const req = new XMLHttpRequest();
req.responseType = 'json';
req.open('GET', '/getdata?id=65');
req.onload = () => {
  console.log(xhr.response);
};
req.send();

POST

const data = {
  fish: 'Salmon',
  weight: '1.5 KG',
  units: 5
};
const xhr = new XMLHttpRequest();
xhr.open('POST', '/inventory/add');
xhr.responseType = 'json';
xhr.send(JSON.stringify(data));
xhr.onload = () => {
  console.log(xhr.response);
};

fetch api

fetch(url, {
    method: 'POST',
    headers: {
      'Content-type': 'application/json',
      'apikey': apiKey
    },
    body: data
  }).then(response => {
    if (response.ok) {
      return response.json();
    }
    throw new Error('Request failed!');
  }, networkError => {
    console.log(networkError.message)
  })
}

JSON 格式

fetch('url-that-returns-JSON')
.then(response => response.json())
.then(jsonResponse => {
  console.log(jsonResponse);
});

promise url 参数获取 API

fetch('url')
.then(
  response  => {
    console.log(response);
  },
 rejection => {
    console.error(rejection.message);
);

Fetch API 函数

fetch('https://api-xxx.com/endpoint', {
  method: 'POST',
  body: JSON.stringify({id: "200"})
}).then(response => {
  if(response.ok){
	  return response.json();  
  }
	throw new Error('Request failed!');
}, networkError => {
  console.log(networkError.message);
}).then(jsonResponse => {
  console.log(jsonResponse);
})

async await syntax

const getSuggestions = async () => {
  const wordQuery = inputField.value;
  const endpoint = `${url}${queryParams}${wordQuery}`;
  try{
    const response = await fetch(endpoint, {cache: 'no-cache'});
    if(response.ok){
      const jsonResponse = await response.json()
    }
  }
  catch(error){
    console.log(error)
  }
}