JavaScript Topic 2: Array Deduplication

Directory

• 1. Double-layer loop (violent method)
• 2. indexOf and includes
• 3. Sorting and deduplication
• 4. Filter
• 5. Key-value pair(key-value)
• 6.ES6
• 7 , some questions
• Reference

• Write at the end

(Related free learning recommendations: javascript video tutorial)

1. Double-layer circulation

const unique = (arr)=>{
    for(let i = 0; i {
    var arr = [1, '1', '1', 1, 2, true, false, true, 3, 2, 2, 1];
    var newArr = [];
    for(let i = 0; i <p><strong>Core point:</strong></p>

• Time complexity: O(n^2)
• The above two methods are two loop traversals, and the processing methods are slightly different
• The above implementation methods are indeed It’s not the best choice, but it has good compatibility~

2. indexOf and includes

2.1 indexOf simplifies one-level loop judgment

Core point:

• If you need to return the original array, you can find the duplicate item in the indexOf method (not equal to the position where it first appeared ) when using splice to remove
• indexOf: Returns the first index where a given element can be found in the array, if not If exists, -1 is returned.
• indexOf(ele, fromIndex)
  • ele: The element to be found
  • fromIndex: The starting position of the element to be found, the default is 0, negative numbers are allowed, -2 means starting from the second to last element
  • Return a subscript (number)

##Code:

const unique = (arr) => {
    var res = [];
    for (let i = 0; i 2.2 includesSimplify one layer of loop judgment<h5></h5><p>Core point:<strong></strong></p>

You can combine it by yourself whether you want to return the original array or a new array~
• includes: Used Determine whether an array contains a specified value. Depending on the situation, if it does, it will return true, otherwise it will return false
##includes(ele, fromIndex)
• ele: The element to be found
  • fromIndex: Start searching at the specified index. The default is 0. If it is a negative value, jump forward by the absolute value of
  fromIndex
  • indexes from the end. . Return result (bool)

Code:

const unique = (arr) => {
    var res = [];
    for (let i = 0; i 

2.3 indexOf and includes selection for the current scene

Here we recommend using includes to find elements:

The return value can be directly used as the conditional statement of if, concise

if(res.indexOf(arr[i]) !== -1 ){ todo }// orif(res.includes(arr[i])){ todo }

Identification

NaNIf there is

NaN

in the array, and you just need to determine whether the array exists NaN, then you use indexOf cannot be judged, you must use the includes method. <pre class="brush:php;toolbar:false">var arr = [NaN, NaN];arr.indexOf(NaN); // -1arr.includes(NaN); // true</pre>

Identification

undefinedIf there is an

undefined

value in the array, includes will think The empty value is undefined, but indexOf will not. <pre class="brush:php;toolbar:false">var arr = new Array(3);console.log(arr.indexOf(undefined)); //-1console.log(arr.includes(undefined)) //true</pre>

3. Sorting and deduplication

Core point:

After the array is sorted, the same elements will Adjacent, so if the current element is different from its adjacent elements, it is stored in a new array;

• Compared with indexOf, only one loop is needed;
• concat will splice the arrays , and returns a new array;
• sort() sorting is done by sorting according to the Unicode position
of each character of the converted
• string. So it is difficult to guarantee its accuracy;

Code:

var arr = [1, 1, '1'];function unique(arr) {
    var res = [];
    var sortedArr = arr.concat().sort();
    var last;
    for (var i = 0; i 

Four, filter

Core point:

filter: method creates a new array containing all elements of the test

implemented by the provided
• function (returns the elements for which the test function is established)
filter(callback, thisArg)
• ：callback accepts three parameters: element-the element currently being processed, index-the current element index, array-the filter was called The array itself
  • thisArg: The value used for this when executing callback.
  Using filter we can simplify the outer loop at the code level:
  • 
    

Code:

var arr = [1, 2, 1, 1, '1'];const unique = function (arr) {
    var res = arr.filter(function(item, index, arr){
        return arr.indexOf(item) === index;
    })
    return res;}console.log(unique(arr)); // [ 1, 2, '1' ]

Combined sorting ideas ：

var arr = [1, 2, 1, 1, '1'];const unique = function (arr) {
    return arr.concat().sort().filter(function(item, index, arr){
        return !index || item !== arr[index - 1]
    })}console.log(unique(arr));

5. Key-value pair

The methods mentioned above can be roughly divided into

Non-sorted array, two traversal judgments (traversal, query)

1. Sorted array, comparison of adjacent elements
2. We propose another way, using the key-value of the Object object Method, to count the number of elements appearing in the array, there are two preliminary judgment logics

Take

[1,1,1,2,2,3,'3']

for example: <ol> <li>统计每个元素出现的次数，obj：{1: 3, 2: 2, 3: 3}, 返回这个<code>obj的key而不管他们的value

只元素首次出现，再次出现则证明他是重复元素

5.1 统计次数

var arr = [1, 2, 1, 1, '1', 3, 3];const unique = function(arr) {
    var obj = {};
    var res = [];
    arr.forEach(item => {
        if (!obj[item]) {
            obj[item] = true;
            res.push(item);
        }
    });
    return res;}console.log(unique(arr)); // [1, 2, 3]

5.2 结合filter

var arr = [1, 2, 1, 1, '1'];const unique = function(arr) {
    var obj = {};
    return arr.filter(function(item, index, arr){
        return obj.hasOwnProperty(item) ? false : (obj[item] = true)
    })}console.log(unique(arr)); // [1, 2]

5.3 key: value存在的问题

对象的属性是字符串类型的，即本身数字1和字符串‘1’是不同的，但保存到对象中时会发生隐式类型转换，导致去重存在一定的隐患。

考虑到string和number的区别（typeof 1 === ‘number’， typeof ‘1’ === ‘string’），

所以我们可以使用 typeof item + item 拼成字符串作为 key 值来避免这个问题：

var arr = [1, 2, 1, 1, '1', 3, 3, '2'];const unique = function(arr) {
    var obj = {};
    var res = [];
    arr.forEach(item => {
        if (!obj[typeof item + item]) {
            obj[typeof item + item] = true;
            res.push(item);
        }
    });
    return res;}console.log(unique(arr)); // [ 1, 2, '1', 3, '2' ]

六、ES6

随着 ES6 的到来，去重的方法又有了进展，比如我们可以使用 Set 和 Map 数据结构。

6.1 Set

Set：它允许你存储任何类型的唯一值，无论是原始值或者是对象引用

代码：

var arr = [1, 2, 1, '1', '2'];const unique = function(arr) {
   return Array.from(new Set(arr));}console.log(unique(arr)); // [ 1, 2, '1', '2' ]

简化1：

function unique(array) {
    return [...new Set(array)];}

简化2：

var unique = (a) => [...new Set(a)]

6.2 Map

Map 对象保存键值对，并且能够记住键的原始插入顺序。任何值(对象或者原始值) 都可以作为一个键或一个值。

• Map.prototype.has(key)：返回一个布尔值，表示Map实例是否包含键对应的值。
• Map.prototype.set(key, value)：设置Map对象中键的值。返回该Map对象。

function unique (arr) {
    const newMap = new Map()
    return arr.filter((a) => !newMap.has(a) && newMap.set(a, 1));}

写到这里比较常规的数组去重方法就总结的差不多了，如果需要更强大的去重方法，我们需要对他们进行组合，而且因为场景的不同，我们所实现的方法并不一定能涵盖到

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