去重，字符出现次数，扁平化数组，数组交集以及并集

1. 去重

let a = [2, 3, 4, 5, 4]
let a1 = [2, 3, 4, 5, 4]
// method1 filter并不会改变原来数组
let method1 = a.filter(function (value, index) {
  return a.indexOf(value) === index
})
console.log('方法-', method1)

// method2
let method2 = [...new Set(a)]
console.log('方法二', method2)

// method3
let method3 = a.reduce(function (a, b) {
  if (a.indexOf(b) == -1) {
    a.push(a)
  }
  return a
}, [])

2. 扁平化数组

let server = require('./server')
server.startServer()
let array = [1, [2, 3, 4, 5, [6, 7, 8, [9, 5, 7, 8], [5, 6, 7]]]]
let c=[];
let resultArray = []
function flatArray (arg) {
  for (let i = 0;i < arg.length;i++) {
    if (Array.isArray(arg[i])) {
      flatArray(arg[i])
    }else {
      resultArray.push(arg[i])
    }
  }
}
flatArray(array);
console.log('递归算法', resultArray);

3. 字符出现次数

function collectLetter(s) {
      let letterarry = s.split("");
      let result = {};
      for (let i of letterarry) {
          result[i] = (result[i] ||0) +1;
      }
      for (let j in result) {
          console.log(`${j} occures ${result[j]}`);
      }
  return result;
  };
  collectLetter("ahdfakjfhkasjfak")

4. 数组的交集以及并集实现

var a=[...new Set([1,2,3]),...new Set([2,3,4])];
console.log("数组并集",a);

var b=[1,2,3].filter(x=>{return [2,3,5,6].indexOf(x)})
console.log("数组交集",b)