How to remove duplicates from js array?

JS array deduplication method: 1. Use the Set object of ES6 to deduplicate; 2. Use for to nest for, and then splice to deduplicate; 3. Use indexOf to deduplicate; 4. Use the properties of the objects not to be the same. Features to remove duplication; 5. Use Map data structure to remove duplication; 6. Use reduce includes to remove duplication.

Method to remove duplicates from array

1. ES6 Set object

ES6 provides a new data structure Set. It is similar to an array, but the values ​​of the members are unique and there are no duplicate values. Set itself is a constructor used to generate the Set data structure.

function unique (arr) {
  return Array.from(new Set(arr))
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,null,
 NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
console.log(unique(arr))
 //[1, "true", true, 15, false, undefined, null, NaN, "NaN", 0, "a", {}, {}]

Regardless of compatibility, this method of deduplication has the least code. This method cannot remove "{}" empty objects, and later high-level methods will add methods to remove repeated "{}".

2. Use for to nest for, and then splice to remove duplicates (most commonly used in ES5)

function unique(arr){            
        for(var i=0; i
Double-layer loop, outer loop element, inner loop Compare values. If the values ​​are the same, this value will be deleted. 

3. Use indexOf to remove duplicates
function unique(arr){            
        for(var i=0; i
Create an empty result array, for loop the original array, and determine whether the current element exists in the result array. If so, If the values ​​are the same, they are skipped; if they are not the same, they are pushed into the array. 
4. Use sort()
function unique(arr) {
    if (!Array.isArray(arr)) {
        console.log('type error!')
        return;
    }
    arr = arr.sort()
    var arrry= [arr[0]];
    for (var i = 1; i < arr.length; i++) {
        if (arr[i] !== arr[i-1]) {
            arrry.push(arr[i]);
        }
    }
    return arrry;
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,
     null, NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
     
console.log(unique(arr))
// [0, 1, 15, "NaN", NaN, NaN, {…}, {…}, "a", false, null, true, "true", undefined]
      //NaN、{}没有去重
Use the sort() sorting method, and then traverse and compare adjacent elements based on the sorted results. 
5. Use the characteristics of objects whose attributes cannot be the same to deduplicate (this method of deduplicating arrays has problems, is not recommended, and needs to be improved) 
function unique(arr) {
    if (!Array.isArray(arr)) {
        console.log('type error!')
        return
    }
    var arrry= [];
     var  obj = {};
    for (var i = 0; i < arr.length; i++) {
        if (!obj[arr[i]]) {
            arrry.push(arr[i])
            obj[arr[i]] = 1
        } else {
            obj[arr[i]]++
        }
    }
    return arrry;
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined,
 null,null, NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
 
console.log(unique(arr))
//[1, "true", 15, false, undefined, null, NaN, 0, "a", {…}]
    //两个true直接去掉了，NaN和{}去重
 6. Use includes
function unique(arr) {
    if (!Array.isArray(arr)) {
        console.log('type error!')
        return
    }
    var array =[];
    for(var i = 0; i < arr.length; i++) {
            if( !array.includes( arr[i]) ) {//includes 检测数组是否有某个值
                    array.push(arr[i]);
              }
    }
    return array
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,null,
 NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
 
console.log(unique(arr))
//[1, "true", true, 15, false, undefined, null, NaN, "NaN", 0, "a", {…}, {…}]
         //{}没有去重
7. Use hasOwnProperty
function unique(arr) {
 var obj = {};
 return arr.filter(function(item, index, arr){
    return obj.hasOwnProperty(typeof item + item) ? false : (obj[typeof item + item] = true)
 })
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,null,
     NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
     
console.log(unique(arr))
//[1, "true", true, 15, false, undefined, null, NaN, "NaN", 0, "a", {…}]
   //所有的都去重了
Use hasOwnProperty to determine whether object properties exist
 8. Use filter
function unique(arr) {
  return arr.filter(function(item, index, arr) {
    //当前元素，在原始数组中的第一个索引==当前索引值，否则返回当前元素
    return arr.indexOf(item, 0) === index;
  });
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,null,
 NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
 
console.log(unique(arr))
//[1, "true", true, 15, false, undefined, null, "NaN", 0, "a", {…}, {…}]
 9. Use recursion to remove duplicates
function unique(arr) {
        var array= arr;
        var len = array.length;

    array.sort(function(a,b){   //排序后更加方便去重
        return a - b;
    })

    function loop(index){
        if(index >= 1){
            if(array[index] === array[index-1]){
                array.splice(index,1);
            }
            loop(index - 1);    //递归loop，然后数组去重
        }
    }
    loop(len-1);
    return array;
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,null,
  NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
  
console.log(unique(arr))
//[1, "a", "true", true, 15, false, 1, {…}, null, NaN, NaN, "NaN", 0, "a", {…}, undefined]
 10. Use Map data structure to remove duplicates
 
function arrayNonRepeatfy(arr) {
  let map = new Map();
  let array = new Array();  // 数组用于返回结果
  for (let i = 0; i < arr.length; i++) {
    if(map .has(arr[i])) {  // 如果有该key值
      map .set(arr[i], true); 
    } else { 
      map .set(arr[i], false);   // 如果没有该key值
      array .push(arr[i]);
    }
  } 
  return array ;
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,null,
  NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
  
console.log(unique(arr))
//[1, "a", "true", true, 15, false, 1, {…}, null, NaN, NaN, "NaN", 0, "a", {…}, undefined]
Create an empty Map data structure, traverse the array that needs to be deduplicated, and store each element of the array as a key into the Map. Since the same key value will not appear in the Map, the final result is the result after deduplication. 
11. Using reduce includes
function unique(arr){
    return arr.reduce((prev,cur) => prev.includes(cur) ? prev : [...prev,cur],[]);
}
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,null,
  NaN, NaN,'NaN', 0, 0, 'a', 'a',{},{}];
  
console.log(unique(arr));
// [1, "true", true, 15, false, undefined, null, NaN, "NaN", 0, "a", {…}, {…}]
12. [...new Set(arr)]
[...new Set(arr)] 
//代码就是这么少----（其实，严格来说并不算是一种，相对于第一种方法来说只是简化了代码）
PS: Some articles mentioned the foreach indexOf array deduplication method. Personally, I think they are all similar, so I didn’t write them down. 
 Related free learning recommendations: js video tutorial
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