Summary of js array deduplication methods_javascript skills

Three methods

Use indexOf to determine the new array

A similar indexOf
is actually used in underscore.js

 //传入数组
 function unique1(arr){
  var tmpArr = [];
  for(var i=0; i<arr.length; i++){
   //如果当前数组的第i已经保存进了临时数组，那么跳过，
   //否则把当前项push到临时数组里面
   if(tmpArr.indexOf(arr[i]) == -1){
    tmpArr.push(arr[i]);
   }
  }
  return tmpArr;
 }

Use indexOf to determine the old array

 function unique2(arr){
  var tmpArr = []; //结果数组
  for(var i=0; i<arr.length; i++){
   //如果当前数组的第i项在当前数组中第一次出现的位置不是i，
   //那么表示第i项是重复的，忽略掉。否则存入结果数组
   if(arr.indexOf(arr[i]) == i){
    tmpArr.push(arr[i]);
   }
  }
  return tmpArr;
 }

Use hash to search

The implementation of JS objects used here is the characteristics of the hash table

 function unique3(arr){
  var tmpArr = [], hash = {};//hash为hash表
  for(var i=0;i<arr.length;i++){
   if(!hash[arr[i]]){//如果hash表中没有当前项
    hash[arr[i]] = true;//存入hash表
    tmpArr.push(arr[i]);//存入临时数组
   }
  }
  return tmpArr;
 }

Array expansion

 Array.prototype.unique1 = function (){
  var tmpArr = []; 
  for (var i = 0; i < this.length; i++){
   if (tmpArr.indexOf(this[i]) == -1){
    tmpArr.push(this[i]);
   }
  }
  return tmpArr;
 }

 Array.prototype.unique2 = function(){
   var tmpArr = []; //结果数组
   for(var i = 0; i < this.length; i++){
    if (this.indexOf(this[i]) == i){
     tmpArr.push(this[i]);
    }
   }
   return tmpArr;
 }

 Array.prototype.unique3 = function(){
   var tmpArr=[], hash = {};
   for(var i = 0; i < this.length; i++){
    if (!hash[this[i]]){
      hash[this[i]] = true; 
      tmpArr.push(this[i]); 
    }
   }
   return tmpArr;
 }

Use Set

Set and Map are new data structures in ES6
Set can directly store a non-duplicate set of keys. This key can also be an object, a string, etc.
Create set

var s = new Set([1, 2, 3,]);
s; // Set {1, 2, 3}

New element

>>> s.add(4)
>>> s
{1, 2, 3, 4}
>>> s.add(4)
>>> s
{1, 2, 3, 4}//重复元素不会被添加

Delete element

s; // Set {1, 2, 3, 4}
s.delete(3);
s; // Set {1, 2, 4}

Traverse elements

Map and Set cannot use subscripts
The ES6 standard introduces a new iterable type. Array, Map and Set all belong to iterable types

var s = new Set(['A', 'B', 'C']);

for (var x of s) { // 遍历Set
  alert(x);
}

Or directly use iterable’s built-in forEach method
The forEach method is introduced by the ES5.1 standard

var s = new Set(['A', 'B', 'C']);
s.forEach(function (element, set) {
  alert(element);
});

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