When you want to write an if...else statement, use the ternary operator instead.
<span style="font-size: 16px;">const x = 20;<br>let answer;<br>if (x > 10) {<br> answer = 'is greater';<br>} else {<br> answer = 'is lesser';<br>}<br></span>Abbreviation: <span style="font-size: 16px;">const answer = x > 10 ? 'is greater' : 'is lesser';</span>
You can also nest if statements: <span style="font-size: 16px;">const big = x > 10 ? " greater 10" : x</span>
When assigning another value to a variable, you want to make sure that the original value is not null, undefined or empty. You can write a multi-condition if statement.
<span style="font-size: 16px;">if (variable1 !== null || variable1 !== undefined || variable1 !== '') {<br> let variable2 = variable1;<br>}<br></span>Or you can use the short-circuit evaluation method: <span style="font-size: 16px;">const variable2 = variable1 || 'new';</span>
<span style="font-size: 16px;">let x;<br>let y;<br>let z = 3;<br></span>
Abbreviated method: <span style="font-size: 16px;">let x, y, z=3 ;</span>
##if (likeJavaScript === true)<span style="font-size: 16px;"></span>
Abbreviation: if (likeJavaScript)<span style="font-size: 16px;"></span>
only when likeJavaScript is true , the two statements are equal
If the judgment value is not a true value, you can do this:
<span style="font-size: 16px;">let a;<br>if ( a !== true ) {<br>// do something...<br>}<br></span>Abbreviation:
<span style="font-size: 16px;">let a;<br>if ( !a ) {<br>// do something...<br>}<br></span>for (let i = 0; i < allImgs.length; i++)<span style="font-size: 16px;"></span>
Abbreviation: for (let index in allImgs)<span style="font-size: 16px;"></span>
You can also use Array.forEach:
<span style="font-size: 16px;">function logArrayElements(element, index, array) {<br/> console.log("a[" + index + "] = " + element);<br/>}<br/>[2, 5, 9].forEach(logArrayElements);<br/>// logs:<br/>// a[0] = 2<br/>// a[1] = 5<br/>// a[2] = 9<br/></span>The value assigned to a variable is determined by judging whether the value is null or undefined, then you can:
<span style="font-size: 16px;">let dbHost;<br/>if (process.env.DB_HOST) {<br/> dbHost = process.env.DB_HOST;<br/>} else {<br/> dbHost = 'localhost';<br/>}<br/></span>Abbreviation: const dbHost = process.env.DB_HOST || 'localhost';<span style="font-size: 16px;"></span>
When you need to write a number with many zeros (such as 10000000), you can use the exponent (1e7) to replace this number: for (let i = 0; i < 10000; i++) {}<span style="font-size: 16px;"></span>
Abbreviation:
<span style="font-size: 16px;">for (let i = 0; i < 1e7; i++) {}<br/><br/>// 下面都是返回true<br/>1e0 === 1;<br/>1e1 === 10;<br/>1e2 === 100;<br/>1e3 === 1000;<br/>1e4 === 10000;<br/>1e5 === 100000;<br/></span>If the attribute name is the same as the key name, you can use the ES6 method: const obj = { x: x, y:y };<span style="font-size: 16px;"></span>
Abbreviation: const obj = { x, y };<span style="font-size: 16px;"></span>
The traditional function writing method is easy for people to understand and write, but when nested in another function, then These advantages are gone.
<span style="font-size: 16px;">function sayHello(name) {<br/> console.log('Hello', name);<br/>}<br/><br/>setTimeout(function() {<br/> console.log('Loaded')<br/>}, 2000);<br/><br/>list.forEach(function(item) {<br/> console.log(item);<br/>});<br/></span>Abbreviation:
<span style="font-size: 16px;">sayHello = name => console.log('Hello', name);<br><br>setTimeout(() => console.log('Loaded'), 2000);<br><br>list.forEach(item => console.log(item));<br></code></p>
<h2>10. Implicit return value abbreviation<span style="font-size: 16px;"></span>
</h2>
<p>Return is often used statement to return the final result of a function, an arrow function with a single statement can implicitly return its value (the function must omit {} in order to omit the return keyword) <span style="font-size: 16px;"></span></p>
<p> is a statement that returns multiple lines (for example Object literal expression), you need to surround the function body with (). <span style="font-size: 16px;"></span></p>
<pre class="brush:php;toolbar:false"><span style="font-size: 16px;">function calcCircumference(diameter) {<br> return Math.PI * diameter<br>}<br><br>var func = function func() {<br> return { foo: 1 };<br>};<br></span>Abbreviation:
<span style="font-size: 16px;">calcCircumference = diameter => (<br> Math.PI * diameter;<br>)<br><br>var func = () => ({ foo: 1 });<br></span>In order to provide parameters in the function Passing default values is usually written using if statements, but using ES6 to define default values will be very concise:
<span style="font-size: 16px;">function volume(l, w, h) {<br> if (w === undefined)<br> w = 3;<br> if (h === undefined)<br> h = 4;<br> return l * w * h;<br>}<br></span>Abbreviation:
<span style="font-size: 16px;">volume = (l, w = 3, h = 4 ) => (l * w * h);<br><br>volume(2) //output: 24<br></span>
In traditional JavaScript language, the output template is usually written like this.
<span style="font-size: 16px;">const welcome = 'You have logged in as ' + first + ' ' + last + '.'<br><br>const db = 'http://' + host + ':' + port + '/' + database;<br></span>
ES6 can use backticks and ${} abbreviation:
<span style="font-size: 16px;">const welcome = `You have logged in as ${first} ${last}`;<br><br>const db = `http://${host}:${port}/${database}`;<br></span>在web框架中，经常需要从组件和API之间来回传递数组或对象字面形式的数据，然后需要解构它
<span style="font-size: 16px;">const observable = require('mobx/observable');<br>const action = require('mobx/action');<br>const runInAction = require('mobx/runInAction');<br><br>const store = this.props.store;<br>const form = this.props.form;<br>const loading = this.props.loading;<br>const errors = this.props.errors;<br>const entity = this.props.entity;<br></span>简写：
<span style="font-size: 16px;">import { observable, action, runInAction } from 'mobx';<br><br>const { store, form, loading, errors, entity } = this.props;<br></span>也可以分配变量名：
<span style="font-size: 16px;">const { store, form, loading, errors, entity:contact } = this.props;<br>//最后一个变量名为contact<br></span>需要输出多行字符串，需要使用+来拼接：
<span style="font-size: 16px;">const lorem = 'Lorem ipsum dolor sit amet, consectetur\n\t'<br> + 'adipisicing elit, sed do eiusmod tempor incididunt\n\t'<br> + 'ut labore et dolore magna aliqua. Ut enim ad minim\n\t'<br> + 'veniam, quis nostrud exercitation ullamco laboris\n\t'<br> + 'nisi ut aliquip ex ea commodo consequat. Duis aute\n\t'<br> + 'irure dolor in reprehenderit in voluptate velit esse.\n\t'<br></span>
使用反引号，则可以达到简写作用：
<span style="font-size: 16px;">const lorem = `Lorem ipsum dolor sit amet, consectetur<br> adipisicing elit, sed do eiusmod tempor incididunt<br> ut labore et dolore magna aliqua. Ut enim ad minim<br> veniam, quis nostrud exercitation ullamco laboris<br> nisi ut aliquip ex ea commodo consequat. Duis aute<br> irure dolor in reprehenderit in voluptate velit esse.`<br></span>
扩展运算符有几种用例让JavaScript代码更加有效使用，可以用来代替某个数组函数。
<span style="font-size: 16px;">// joining arrays<br>const odd = [1, 3, 5];<br>const nums = [2 ,4 , 6].concat(odd);<br><br>// cloning arrays<br>const arr = [1, 2, 3, 4];<br>const arr2 = arr.slice()<br></span>
简写：
<span style="font-size: 16px;">// joining arrays<br>const odd = [1, 3, 5 ];<br>const nums = [2 ,4 , 6, ...odd];<br>console.log(nums); // [ 2, 4, 6, 1, 3, 5 ]<br><br>// cloning arrays<br>const arr = [1, 2, 3, 4];<br>const arr2 = [...arr];<br></span>
不像concat()函数，可以使用扩展运算符来在一个数组中任意处插入另一个数组。
<span style="font-size: 16px;">const odd = [1, 3, 5 ];<br>const nums = [2, ...odd, 4 , 6];<br></span>
也可以使用扩展运算符解构：
<span style="font-size: 16px;">const { a, b, ...z } = { a: 1, b: 2, c: 3, d: 4 };<br>console.log(a) // 1<br>console.log(b) // 2<br>console.log(z) // { c: 3, d: 4 }<br></span>JavaScript中如果没有向函数参数传递值，则参数为undefined。为了增强参数赋值，可以使用if语句来抛出异常，或使用强制参数简写方法。
<span style="font-size: 16px;">function foo(bar) {<br> if(bar === undefined) {<br> throw new Error('Missing parameter!');<br> }<br> return bar;<br>}<br></span>简写：
<span style="font-size: 16px;">mandatory = () => {<br> throw new Error('Missing parameter!');<br>}<br><br>foo = (bar = mandatory()) => {<br> return bar;<br>}<br></span>想从数组中查找某个值，则需要循环。在ES6中，find()函数能实现同样效果。
<span style="font-size: 16px;">const pets = [<br> { type: 'Dog', name: 'Max'},<br> { type: 'Cat', name: 'Karl'},<br> { type: 'Dog', name: 'Tommy'},<br>]<br><br>function findDog(name) {<br> for(let i = 0; i<pets.length; ++i) {<br/> if(pets[i].type === 'Dog' && pets[i].name === name) {<br/> return pets[i];<br/> }<br/> }<br/>}<br/></span>简写：
<span style="font-size: 16px;">pet = pets.find(pet => pet.type ==='Dog' && pet.name === 'Tommy');<br>console.log(pet); // { type: 'Dog', name: 'Tommy' }<br></span>考虑一个验证函数
<span style="font-size: 16px;">function validate(values) {<br> if(!values.first)<br> return false;<br> if(!values.last)<br> return false;<br> return true;<br>}<br><br>console.log(validate({first:'Bruce',last:'Wayne'})); // true<br></span>假设当需要不同域和规则来验证，能否编写一个通用函数在运行时确认？
<span style="font-size: 16px;">// 对象验证规则<br>const schema = {<br> first: {<br> required:true<br> },<br> last: {<br> required:true<br> }<br>}<br><br>// 通用验证函数<br>const validate = (schema, values) => {<br> for(field in schema) {<br> if(schema[field].required) {<br> if(!values[field]) {<br> return false;<br> }<br> }<br> }<br> return true;<br>}<br><br><br>console.log(validate(schema, {first:'Bruce'})); // false<br>console.log(validate(schema, {first:'Bruce',last:'Wayne'})); // true<br></span>现在可以有适用于各种情况的验证函数，不需要为了每个而编写自定义验证函数了
有一个有效用例用于双重非运算操作符。可以用来代替Math.floor()，其优势在于运行更快，可以阅读此文章了解更多位运算。<span style="font-size: 16px;">Math.floor(4.9) === 4 //true</span>
简写：<span style="font-size: 16px;">~~4.9 === 4 //true</span>
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