var a = [4,2,5,6,3,4,5]
 var hashTab = {}
 for(let i=0; i<a.length;i++){
     if(a[i] in hashTab){
         // 什么也不做
     }else{
         hashTab[ a[i] ] = true
     }
 }
 //hashTab: {4: true, 2: true, 5: true, 6:true, 3: true}
 console.log(Object.keys(hashTab)) // ['4','2','5','6','3']

var array=[4,5,4,7,2,4,6,7]
var result=array.sort().reduce((acc,cur)=>{
  if(acc[acc.length-1]!==cur){
    acc.push(cur)
  }
  return acc
},[])
console.log(result)

function unique(arr){
  var result=[]
arr.map(el=>{
    if(result.indexOf(el)===-1){
      result.push(el)
    }
  })
   return result
}
var a=[3,5,4,6,5,3,5,6,2]
console.log(unique(a))