模板字符串

let name = 'siri', age = 18, job = 'front-end engineer'
let oldStr = 'Hi,' + name + ', I\'m ' + age + 'and work as a' + job + '.';

let newStr = Hi, <span class="hljs-subst">${ name }</span>, I'm <span class="hljs-subst">${ age }</span> and work as a <span class="hljs-subst">${ job }</span>.;

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扩展操作符

… 操作符,有两个主要用处:

  1. 复制一个新的数组或对象
  2. 把多个参数赋值给一个数组变量
  3. 把一个数组变量赋值给多个参数
let a = [1, 2, 3]
let b = [...a] // b 是一个新的数组,内容和 a 一样
let c = [...a, 4, 5, 6]

let car = { type: ’vehicle ’, wheels: 4};
let newCar = {…car}
console.log(newCar); // {type: ’vehicle ’, wheels: 4}

// 合并对象属性,后边的属性会覆盖前边的,可用于修改对象的某个属性值
let car2 = {…car, type: ‘vehicle2’, wheels: 2} // {type: “vehicle2”, wheels: 2}

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function foo(...args) {
    console.log(args); 
} 
foo( 'car', 54, 'tree');  //  console.log 输出 ['car', 54, 'tree'] 
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默认参数

// 给方法添加默认参数值
function foo( a = 5, b = 10) {
    console.log( a + b);
} 
foo();  // 15
foo( 7, 12 );  // 19
foo( undefined, 8 ); // 13
foo( 8 ); // 18
foo( null ); // 10 as null is coerced to 0
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// 默认参数值也可以是表达式或者函数
function foo( a ) { return a * 4; }

// y = x + 4, z = foo(x)
function bar( x = 2, y = x + 4, z = foo(x)) {
console.log([x, y, z]);
}

bar(); // [2, 6, 8]
bar( 1, 2, 3 ); //[1, 2, 3]
bar( 10, undefined, 3 ); // [10, 14, 3]

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// 对象参数默认值,如果参数为空,则会抛出异常
function show({ title = "title", width = 100, height = 200 }) {
  console.log( `${title} ${width} ${height}` );
}
show() // Cannot destructure property `title` of 'undefined' or 'null'.
show({}) // title 100 200

// 解决办法:
function show({ title = “title”, width = 100, height = 200 } = {}) {
console.log( <span class="hljs-subst">${title}</span> <span class="hljs-subst">${width}</span> <span class="hljs-subst">${height}</span> );
}

show(); // title 100 200
show({width: 200}) // title 200 200

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解析赋值

// key 变量重命名, first --> firstName
const person = {
  first: 'foo',
  last: 'tom',
};

const { first: firstName } = person;
console.log(firstName); // foo

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// 默认值
const settings = {
    speed: 150
}
const { speed = 750, width = 500 } = settings;
console.log(speed); // 150 
console.log(width); // 500

// 可能不存在的 key
const { middle: middleName = ‘midname’ } = person;
console.log(middleName); // ‘midname’

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// 嵌套赋值
const user = {
  id: 339,
  name: 'Fred',
  age: 42,
  education: {
    degree: 'Masters'
  }
};
const {education: {degree}} = user;
console.log(degree); //prints: Masters
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// 如果嵌套的属性不存在
const user = {
  id: 339,
  name: 'Fred',
  age: 42
};
const {education: {degree}} = user;  // TypeError: Cannot match against 'undefined' or 'null'.

// 解决办法:
const user = {
id: 339,
name: ‘Fred’,
age: 42
};
const {education: {degree} = {}} = user;
console.log(degree); //prints: undefined

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利用数组生成一个数字序列

const numRange = (start, end) => {
  return Array(end - start + 1).fill().map((item, index) => start + index);
};

const numbers = numRange(0, 5); // [0, 1, 2, 3, 4, 5]
const numbers2 = numRange(1, 5); // [1, 2, 3, 4, 5]

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利用 Set 给数组去重

const years = [2016, 2017, 2017, 2018, 2018, 2019]

// set 构造函数的参数是一个 array
const distinctYears = […new Set(years)] // [2016, 2017, 2018, 2019]

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生成唯一随机字符串,可以指定长度

function generateRandom(length) {
	let radom13chars = function () {
		return Math.random().toString(16).substring(2, 15)
	}
	let loops = Math.ceil(length / 13)
	return new Array(loops).fill(radom13chars).reduce((string, func) => {
		return string + func()
	}, '').substring(0, length)
}

generateRandom(8) // “03836a49”

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