Several methods of array deduplication

1. Traverse array method

It is the easiest way to deduplicate an array (indexOf method)

Implementation idea: Create a new array, traverse the array to be repeated, and add it to the new array when the value is not in the new array (indexOf is -1);

var arr=[2,8,5,0,5,2,6,7,2];
function unique1(arr){
var hash=[];
for (var i = 0; i < arr.length; i++) {
if(hash.indexOf(arr[i])==-1){
hash.push(arr[i]);
}
}
return hash;
}

2. Array subscript judgment method

Call the indexOf method, the performance is similar to method 1

Implementation idea: If the first occurrence of the i-th item of the current array is not i in the current array, it means that the i-th item is repeated and ignored. Otherwise, store the result array.

function unique2(arr){
var hash=[];
for (var i = 0; i < arr.length; i++) {
if(arr.indexOf(arr[i])==i){
hash.push(arr[i]);
}
}
return hash;
}

3. Adjacent removal method after sorting 

Implementation idea: sort the incoming array, the same values ​​will be adjacent after sorting, and then when traversing the sorted array, the new array will only add values ​​that do not duplicate the previous value.

function unique3(arr){
arr.sort();
var hash=[arr[0]];
for (var i = 1; i < arr.length; i++) {
if(arr[i]!=hash[hash.length-1]){
hash.push(arr[i]);
}
}
return hash;
}

4. Optimize traversal array method (recommended)

Implementation idea: double-layer loop, the outer loop represents from 0 to arr.length, and the inner loop represents from i+1 to arr.length

Put the non-repeated right-hand values ​​into a new array. (When a duplicate value is detected, the current loop is terminated and the next round of judgment of the outer loop is entered)

function unique4(arr){
var hash=[];
for (var i = 0; i < arr.length; i++) {
for (var j = i+1; j < arr.length; j++) {
if(arr[i]===arr[j]){
++i;
}
}
hash.push(arr[i]);
}
return hash;
}

5. ES6 implementation

Basic idea: ES6 provides a new data structure Set. It is similar to an array, but the values ​​of the members are all unique and there are no duplicate values.

The Set function can accept an array (or array-like object) as a parameter for initialization.

function unique5(arr){
  var x = new Set(arr);
 return [...x];
}

extension: if duplicate, remove the element

Array subscript deduplication

function unique22(arr){
var hash=[];
for (var i = 0; i < arr.length; i++) {
if(arr.indexOf(arr[i])==arr.lastIndexOf(arr[i])){
hash.push(arr[i]);
}
}
return hash;
}