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javascript 备忘单
2021-08-30 14:07:10 更新
包含最重要的概念、函数、方法等的 JavaScript 备忘单。初学者的完整快速参考。
开始
介绍
JavaScript 是一种轻量级的解释型编程语言。
- 在 Y 分钟内学习 X (learnxinyminute.com)
- JavaScript 中的正则表达式 (quickref.me)
console.log()
alert('Hello world!');
console.log('Hello world!');
// => Hello world!
数值
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 // Addition
10 - 5 = 5 // Subtraction
5 * 10 = 50 // Multiplication
10 / 5 = 2 // Division
10 % 5 = 0 // Modulo
注释
// This line will denote a comment
/*
The below configuration must be
changed before deployment.
*/
赋值运算符
let number = 100;
// Both statements will add 10
number = number + 10;
number += 10;
console.log(number);
// => 120
字符串插值
let age = 7;
// String concatenation
'Tommy is ' + age + ' years old.';
// String interpolation
`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 语句
const isMailSent = true;
if (isMailSent) {
console.log('Mail sent to recipient');
}
三元运算符
var x=1;
// => true
result = (x == 1) ? true : false;
运算符
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);
否则如果
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('The taste of the sea');
break;
case 'pizza':
console.log('A delicious pie');
break;
default:
console.log('Enjoy your meal');
}
JavaScript 函数
函数
// Defining the function:
function sum(num1, num2) {
return num1 + num2;
}
// Calling the function:
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)
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
一个参数
const checkWeight = weight => {
console.log(`Weight : ${weight}`);
};
checkWeight(25); // => Weight : 25
简洁的箭头函数
const multiply = (a, b) => a * b;
// => 60
console.log(multiply(2, 30));
返回关键字
// With return
function sum(num1, num2) {
return num1 + num2;
}
// The function doesn't output the sum
function sum(num1, num2) {
num1 + num2;
}
调用函数
// Defining the function
function sum(num1, num2) {
return num1 + num2;
}
// Calling the function
sum(2, 4); // 6
函数表达式
const dog = function() {
return 'Woof!';
}
功能参数
// The parameter is name
function sayHello(name) {
return `Hello, ${name}!`;
}
函数声明
function add(num1, num2) {
return num1 + num2;
}
JavaScript 范围
范围
function myFunction() {
var pizzaName = "Volvo";
// Code here can use pizzaName
}
// Code here can't use pizzaName
块作用域变量
const isLoggedIn = true;
if (isLoggedIn == true) {
const statusMessage = 'Logged in.';
}
// Uncaught ReferenceError...
console.log(statusMessage);
全局变量
// Variable declared globally
const color = 'blue';
function printColor() {
console.log(color);
}
printColor(); // => blue
JavaScript 数组
数组
const a1 = [0, 1, 2, 3];
// Different data types
const a2 = [1, 'chicken', false];
属性 .length
const numbers = [1, 2, 3, 4];
numbers.length // 4
指数
// Accessing an array element
const myArray = [100, 200, 300];
console.log(myArray[0]); // 100
console.log(myArray[1]); // 200
方法 .push()
// Adding a single element:
const cart = ['apple', 'orange'];
cart.push('pear');
// Adding multiple elements:
const numbers = [1, 2];
numbers.push(3, 4, 5);
方法 .pop()
const a= ['eggs', 'flour', 'chocolate'];
const p = a.pop(); // 'chocolate'
console.log(a); // ['eggs', 'flour']
可变的
const names = ['Alice', 'Bob'];
names.push('Carl');
// ['Alice', 'Bob', 'Carl']
JavaScript 循环
While 循环
while (condition) {
// code block to be executed
}
let i = 0;
while (i < 5) {
console.log(i);
i++;
}
反向循环
const a = ['banana', 'cherry'];
for (let i = a.length - 1; i >= 0; i--){
console.log(`${i}. ${items[i]}`);
}
// => 2. cherry
// => 1. banana
Do...while 语句
x = 0
i = 0
do {
x = x + i;
console.log(x)
i++;
} while (i < 5);
// => 0 1 3 6 10
For循环
for (let i = 0; i < 4; i += 1) {
console.log(i);
};
// => 0, 1, 2, 3
循环遍历数组
for (let i = 0; i < array.length; i++){
console.log(array[i]);
}
// => Every item in the array
Break
for (let i = 0; i < 99; i += 1) {
if (i > 5) {
break;
}
console.log(i)
}
// => 0 1 2 3 4 5
Continue
for (i = 0; i < 10; i++) {
if (i === 3) { continue; }
text += "The number is " + i + "<br>";
}
嵌套循环
for (let i = 0; i < 2; i += 1) {
for (let j = 0; j < 3; j += 1) {
console.log(`${i}-${j}`);
}
}
for...in 循环
let dic = {brand: 'Apple', model: ''};
for (let key in mobile) {
console.log(`${key}: ${mobile[key]}`);
}
JavaScript 迭代器
分配给变量的函数
let plusFive = (number) => {
return number + 5;
};
// f is assigned the value of plusFive
let f = plusFive;
plusFive(3); // 8
// Since f has a function value, it can be invoked.
f(9); // 14
回调函数
const isEven = (n) => {
return n % 2 == 0;
}
let printMsg = (evenFunc, num) => {
const isNumEven = evenFunc(num);
console.log(`${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 arrayOfNumbers = [1, 2, 3, 4];
const sum = arrayOfNumbers.reduce((accumulator, curVal) => {
return accumulator + curVal;
});
console.log(sum); // 10
数组方法 .map()
const a = ['Taylor', 'Donald', 'Don', 'Natasha', 'Bobby'];
const announcements = a.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
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
// Example of invalid key names
const trainSchedule = {
// Invalid because of the space between words.
platform num: 10,
// Expressions cannot be keys.
40 - 10 + 2: 30,
// A + sign is invalid unless it is enclosed in quotations.
+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: Assignment to constant variable.
赋值速记语法
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);
// Will output 8 since integers are passed by value.
console.log(origNum);
// Will output 'red' since objects are passed
// by reference and are therefore mutable.
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
// A factory function that accepts 'name',
// 'age', and 'breed' parameters to return
// a customized dog object.
const dogFactory = (name, age, breed) => {
return {
name: name,
age: age,
breed: breed,
bark() {
console.log('Woof!');
}
};
};
方法
const engine = {
// method shorthand, with one argument
start(adverb) {
console.log(`The engine starts up ${adverb}...`);
},
// anonymous arrow function expression with no arguments
sputter: () => {
console.log('The engine sputters...');
},
};
engine.start('noisily');
engine.sputter();
Getters and setters
const myCat = {
_name: 'Dottie',
get name() {
return this._name;
},
set name(newName) {
this._name = newName;
}
};
// Reference invokes the getter
console.log(myCat.name);
// Assignment invokes the setter
myCat.name = 'Yankee';
JavaScript 类
静态方法
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();
类构造函数
class Song {
constructor(title, artist) {
this.title = title;
this.artist = artist;
}
}
const mySong = new Song('Bohemian Rhapsody', 'Queen');
console.log(mySong.title);
类方法
class Song {
play() {
console.log('Playing!');
}
stop() {
console.log('Stopping!');
}
}
延伸
// 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 模块
Require
var moduleA = require( "./module-a.js" );
// The .js extension is optional
var moduleA = require( "./module-a" );
// Both ways will produce the same result.
// Now the functionality of moduleA can be used
console.log(moduleA.someFunctionality)
Export
// module "moduleA.js"
export default function cube(x) {
return x * x * x;
}
// In main.js
import cube from './moduleA.js';
// Now the `cube` function can be used straightforwardly.
console.log(cube(3)); // 27
导出模块
let Course = {};
Course.name = "Javascript Node.js"
module.exports = Course;
导入关键字
// add.js
export const add = (x, y) => {
return x + y
}
// main.js
import { add } from './add';
console.log(add(2, 3)); // 5
JavaScript 承诺
承诺状态
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) => alert(err));
执行器函数
const executorFn = (resolve, reject) => {
resolve('Resolved!');
};
const promise = new Promise(executorFn);
setTimeout()
const loginAlert = () =>{
alert('Login');
};
setTimeout(loginAlert, 6000);
.then() 方法
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('Result');
}, 200);
});
promise.then((res) => {
console.log(res);
}, (err) => {
alert(err);
});
.catch() 方法
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
reject(Error('Promise Rejected Unconditionally.'));
}, 1000);
});
promise.then((res) => {
console.log(value);
});
promise.catch((err) => {
alert(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]);
});
避免嵌套 Promise 和 .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);
创建
const executorFn = (resolve, reject) => {
console.log('The executor function of the promise!');
};
const promise = new Promise(executorFn);
链接多个 .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) => {
alert(err)
});
JavaScript 异步等待
异步
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
解决承诺
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"]
异步等待承诺
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
错误处理
let json = '{ "age": 30 }'; // incomplete data
try {
let user = JSON.parse(json); // <-- no errors
alert( user.name ); // no name!
} catch (e) {
alert( "Invalid JSON data!" );
}
Aysnc 等待运算符
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
};
XMLHttpRequest
const xhr = new XMLHttpRequest();
xhr.open('GET', 'mysite.com/getjson');
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);
};
获取 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);
});
承诺url参数获取api
fetch('url')
.then(
response => {
console.log(response);
},
rejection => {
console.error(rejection.message);
);
获取 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);
})
异步等待语法
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)
}
}