数组去重

1.indexOf

function unique(arr) {
    let ret = []
  let current
  for(let i = 0;i<arr.length;i++) {
    current = arr[i]
      if(ret.indexOf(current) === -1) {
        ret.push(current)
    }
  }
  return ret
}

2.排序后去重

function unique(arr) {
    let ret = []
  let sortedArr = arr.concat().sort()
  let previous
  for(let i = 0;i<sortedArr.length;i++) {
      if(!i || sortedArr[i] !== previous ) {
        ret.push(sortedArr[i])
    }
    previous = sortedArr[i]
  }
  return ret
}

3.合并indexOf和排序

function uniqueAPI(arr,isSorted) {
    let ret = []
  let previous
  for(let i = 0;i<arr.length;i++) {
      if(isSorted) {
        if(!i || arr[i] !== previous) {
          ret.push(arr[i])
      }
      previous = arr[i]
    }else {
        if(ret.indexOf(arr[i]) === -1) {
          ret.push(arr[i])
      }
    }
  }
  return ret
}

4.filter

4.1 indexOf简化外层循环

function unique(arr) {
    arr.filter((item,index,arr) => {
      return item.indexOf(arr) === index
  })
}

4.2 sort简化外层循环

function unique(arr) {
    arr.concat().sort().filter((item,index,arr) => {
      return !index || item !== arr[index-1]
  })
}

4.3 对象键值对

function unique(arr) {
    let obj = {}
  arr.filter((item,index,arr) => {
      obj.hasOwnProperty(typeof item + item)? false: (obj[typeof item + item] = true)
  })
}

5.ES6

5.1 Set

let unique = arr => [...new Set(arr)]

5.2 Map

function unique(arr) {
    let map = new Map()
  arr.filter(item =>  !map.has(item) && map.set(item,1) )
}