ES6高级技巧(三)

html转译

let c='<a href="#">Me & you</a>'.replace(/[&<>'"]/g,item=>{
  let a={
    '&': '&amp;',
    '<': '&lt;',
    '>': '&gt;',
    "'": '&#39;',
    '"': '&quot;'
  };
  return a[item]||item
  }
)
console.log(c)

数字前面加密

const mask = (cc, num = 4, mask = '*') => `${cc}`.slice(-num).padStart(`${cc}`.length, mask);

mask(1234567890); // '******7890'
mask(1234567890, 3); // '*******890'
mask(1234567890, -4, '$'); // '$$$$567890'

字符串倒序

const reverseString = str => [...str].reverse().join('');

reverseString('foobar'); // 'raboof'

字符串升序

localeCompare 可以用来判断升序还是降序

const sortCharactersInString = str =>
[...str].sort((a, b) => a.localeCompare(b)).join('');

字符串 数组交换位置

字符串

const stringPermutations = str => {
  if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
  return str
    .split('')
    .reduce(
      (acc, letter, i) =>
        acc.concat(stringPermutations(str.slice(0, i) + str.slice(i + 1)).map(val => letter + val)),
      []
    );
};

stringPermutations('abc'); // ['abc','acb','bac','bca','cab','cba']

数组

const permutations = arr => {
  if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
  return arr.reduce(
    (acc, item, i) =>
      acc.concat(
        permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [item, ...val])
      ),
    []
  );
};

promise一种写法

let a=Promise.resolve([1,2,3,4]).then(res=>res.map(v=>v*5))

a.then(res=>{
  console.log(res)
  })

事件委托的另一种方式填充数据

<div id="a"></div>
<script>
  let a = document.querySelector('#a')
  let arr = ['1', '2']
  a.innerHTML+=arr.map(val=>`<li>${val}</li>`).join('')
</script>

正则

不能是开头：(?<!^)
三个数字：(\d{3})
后面要正好是3的整数倍个数字：(?=(\d{3})*$)
合起来就是 /(?<!^)(\d{3})(?=(\d{3})*$)/g，我们需要一个g表示全局匹配，测试一下：
const newRegexp = /(?<!^)(\d{3})(?=(\d{3})*$)/g
const numStr = '1234567890'
console.log(numStr.match(newRegexp))
// ["234", "567", "890"]

in

如果是数组,返回索引

如果是对象,返回属性

算法和数据结构

资料

在sort()方法中,V8引擎对sort方法执行小于10个使用插入排序,大于十个使用快速排序

对于多个条件,使用Array.includes

const checkCarModel = (model) => {
    if(model === 'renault' || model === 'peugeot') { 
    console.log('model valid');
    }
}

checkCarModel('renault'); // 输出 'model valid'

修改版
const checkCarModel = (model) => {
    if(['peugeot', 'renault'].includes(model)) { 
    console.log('model valid');
    }
}

匹配所有条件,使用Array.every 或者Array.find

const checkEveryModel = (model) => {
  return cars.every(car => car.model === model);
}

const checkEveryModel = (model) => {
  return cars.find(car => car.model !== model) === undefined;
}

匹配部分条件

const cars = [
  { model: 'renault', year: 1956 },
  { model: 'peugeot', year: 1968 },
  { model: 'ford', year: 1977 }
];

const checkForAnyModel = (model) => {
  return cars.some(car => car.model === model);
}

如何避免if多层分支

先考虑不符合条件的情况

如果A不符合的,B不符合的,A&&B不符合的

使用索引或者映射,而不是switch

反例

const getCarsByState = (state) => {
  switch (state) {
    case 'usa':
      return ['Ford', 'Dodge'];
    case 'france':
      return ['Renault', 'Peugeot'];
    case 'italy':
      return ['Fiat'];
    default:
      return [];
  }
}

修改版

const cars = new Map()
  .set('usa', ['Ford', 'Dodge'])
  .set('france', ['Renault', 'Peugeot'])
  .set('italy', ['Fiat']);

const getCarsByState = (state) => {
  return cars.get(state) || [];
}

console.log(getCarsByState()); // 输出 []
console.log(getCarsByState('usa')); //输出 ['Ford', 'Dodge']
console.log(getCarsByState('italy')); // 输出 ['Fiat']

或者(这个很牛逼)

const carState = {
  usa: ['Ford', 'Dodge'],
  france: ['Renault', 'Peugeot'],
  italy: ['Fiat']
};

const getCarsByState = (state) => {
  return carState[state] || [];
}

console.log(getCarsByState()); // 输出 []
console.log(getCarsByState('usa')); // 输出 ['Ford', 'Dodge']
console.log(getCarsByState('france')); // 输出 ['Renault', 'Peugeot']

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