Sharing several methods to remove duplicates from JavaScript arrays

Array deduplication, the general requirement is to give you an array, call the deduplication method, and return a copy of the value. There will be no duplicate elements in the copy. Generally speaking, two elements that return true through === comparison are considered to be the same element and need to be deduplicated. Therefore, 1 and "1" are different elements, and 1 and new Number(1) are different elements, {} and {} are different elements (different references). (Of course, if the requirement is that {} and {} count as the same elements, then the solution will be different). This article mainly shares with you several methods for deduplicating JavaScript arrays.

method 1

Use double loop

function unique(arr) {
 var res = [];
 for(var i = 0, len = arr.length;i < len; i++) {
  var item = arr[i];
  for(var j = 0, jLen = res.length; j
method 2
function unique(arr) {
 var ret = []
 for (var i = 0; i < arr.length; i++) {
 var item = arr[i]
 if (ret.indexOf(item) === -1) {
  ret.push(item)
 }
 }
 return ret
}
You can use a syntax sugar to judge here
function unique(arr) {
 var res = [];
 for(var i = 0, len = arr.length;i < len; i++) {
  var item = arr[i];
  (res.indexOf(item) === -1) && res.push(item);
 }
 return res;
}
But in Lower version browsers do not have indexOf
var indexOf = [].indexOf ?
 function(arr, item) {
  return arr.indexOf(item)
 } :
 function indexOf(arr, item) {
  for (var i = 0; i < arr.length; i++) {
  if (arr[i] === item) {
   return i
  }
  }
  return -1
 }
function unique(arr) {
 var ret = []
 for (var i = 0; i < arr.length; i++) {
 var item = arr[i]
 if (indexOf(ret, item) === -1) {
  ret.push(item)
 }
 }
 return ret
}
method3
Another comparison method using double loops. The previous one is to compare the elements of the original array and the result array one by one. Below we can Put the last element of the repeated elements of the original array into the array
function unique(arr) {
 var ret = [];
 var len = arr.length;
 var isRepeat;
 for(var i=0; i
There is also an optimized version
function unique(a) {
 var res = [];
 for (var i = 0, len = a.length; i < len; i++) {
 for (var j = i + 1; j < len; j++) {
  // 这一步十分巧妙
  // 如果发现相同元素
  // 则 i 自增进入下一个循环比较
  if (a[i] === a[j])
  j = ++i; //j = i = i + 1;
 }
 res.push(a[i]);
 }
 return res;
}
method4
Use the object object in javascript as Hash table
function dedup(arr) {
 var hashTable = {};
 return arr.filter(function(value,index,arr){
  var key = JSON.stringify(value);
  var match = Boolean(hashTable[key]);
  return (match ? false : hashTable[key] = true);
 });
}
Because the key values ​​​​of Object are all String types, 1 and "1" cannot be distinguished. We can make a slight improvement and store the type in the key
function dedup(arr) {
 var ret = [];
 var hash = {};
 for(var i = 0; i < arr.length; i++) {
  var item = arr[i];
  var key = typeof(item) + item;
  if(hash[key] !== 1) {
   ret.push(item)
   hash[key] = 1;
  }
 }
 return ret;
}
 Related recommendations: 
PHP method code for array deduplication
JS simple method analysis for array deduplication
 jQuery simple implementation of array deduplication and sorting operations detailed explanation
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