Array de duplication array sorting maximum sum class array conversion filter array

One 、 Flatten nested arrays / Flattening and arraying holes ——flat()

1、 Realization effect

var arr1 = [1, 2, [3, 4]];  
   arr1.flat(); // [1, 2, 3, 4] 
   var arr2 = [1, 2, [3, 4, [5, 6]]]; 
   arr2.flat(2); // [1, 2, 3, 4, 5, 6]
   //flat Method to delete the empty slot in the array ：
   var arr3 = [1, 2, , 4, 5]; 
   arr3.flat(); // [1, 2, 4, 5]

The flat([depth]) Method to create a new array , All subarray elements are recursively connected to a specified depth .depth The depth level specifies how deep the nested array structure should be flattened . The default is 1.

2、 Reading, ：

The essence of this method is to use recursion and array merging method concat Make it flat . have access to

Array.reduce()/Array.some()/Array.concat()

utilize Array.reduce() and Array.concat() Realization

 const flat1 = function (arr) {
        return arr = arr.reduce((pre, cur) => Array.isArray(cur) ? pre.concat(flat1(cur)) : pre.concat(cur),
        [])
    }
    var arr3 = [1, 2, 3, [1, 2, 3, 4, [2, 3, 4]]];
    flat1(arr3); // [1, 2, 3, 1, 2, 3, 4, 2, 3, 4]

utilize Array.some() and Array.concat() Realization

const flat2 = function(arr){
      while(arr.some(item=>Array.isArray(item))){
          arr=[].concat(...arr);
      }
      return arr;
    }
    var arr3 = [1, 2, 3, [1, 2, 3, 4, [2, 3, 4]]];
    flat2(arr3); // [1, 2, 3, 1, 2, 3, 4, 2, 3, 4]

Two 、 Array weight removal ——set()

1、 Realization effect

Array.from(new Set([1,2,3,3,4,4])) //[1,2,3,4] 
[...new Set([1,2,3,3,4,4,5,6])] //[1,2,3,4,5]

set yes ES6 A new type of data that defines non repeating arrays .
Array.from Is to convert a class array to an array .
... Is the extension operator , take set The values in it are converted to strings .

2、 Reading,

function unique(arr) {
        var result =[];
        for (var i = 0; i < arr.length; i++) {
            for (var j = i+1; j < arr.length; j++) {
                if (arr[i] === arr[j]) {
                   j=++i;
                }

            }
            result.push(arr[i]);
        }
       return result;
    }
    var arr3 = [1, 2, 3, 3, 4, 5, 4];
    unique(arr3);//(5) [1, 2, 3, 5, 4]

3、 ... and 、 Array sorting ——sort()

1、 Realization effect

[1,3,2,5,4].sort(); 
[1,3,2,5,4].sort((a,b)=>b-a);

2、 Reading,

Bubble sort   

Array.prototype.bulesort = function () {
 let arr = this,
     len = arr.length;
 for (let outer = len; outer >= 2; outer--) {
    for (let inner = 0; inner <= outer - 1; inner++) {
        if (arr[inner] > arr[inner + 1]) {
            [arr[inner], arr[inner + 1]] = [arr[inner + 1], arr[inner]];
        }
    }
}
  return arr; 
}
[3, 4, 2,1].bulesort()//[1,2,3,4]

Four 、 Maximum

1、 Realization effect

Math.max(...[1,2,3,4]); //4
Math.max.apply(this, [1, 2, 3, 4]); //4
[1, 2,3, 4].reduce((prev,cur)=>{
return Math.max(prev, cur); 
},0);//4

2、 Reading,
Prioritize , Then take .

5、 ... and 、 Sum up

1、 Realization effect

[1, 2, 3, 4].reduce((prev,cur)=>{
return prev+cur; 
},0); //10

2、 Reading,

function sum(arr){
 if (arr.length==0){
  return 0;
} else if (arr.length == 1){
    return arr[0];    
}else{
 return arr[0]+sum(arr.slice(1));   
 } 
} 
sum([1, 2, 3, 4]);//10

6、 ... and 、 Merge

[1,2,3].concat([5,6]); 
[...[1,2,4],...[2,3,5,7]]

7、 ... and 、 Judge whether there is value

[2, 3, 5].indexOf(3);//1 
[2,3,5].includes(4)//false 
[2, 3, 5].find((item) => { item === 3})//1 
[2, 3, 5].findIndex((item) => item === 3)//1 
[2, 3, 5].some(item=>{return item===3 })//true

8、 ... and 、 Class array conversion

Array.prototype.slice.call(arguments);
Array.prototype.slice.apply(arguments);
Array.from(arguments);
[...arguments]

Nine 、 Each setting value

//array.fill(value, start, end) 
[2, 3, 5].fill(true,false,true);//(3)
[false, false, false] [2, 3, 5].map(() => 0)//(3) [0, 0, 0]

Ten 、 Whether each item satisfies / Whether a certain item satisfies

[2, 3, 5].every(item=>{return item>2}) //false 
[2, 3, 5].some(item =>{ return item > 2 })//true

11、 ... and 、 Filter array

[2, 3, 5].filter(item => { return item > 2 })//(2) [3, 5]

Twelve 、 Object and array conversion

Object.keys({ name: ' Zhang San ', age: 14 })//["name", "age"] 
Object.values({name: ' Zhang San ', age: 14 })// [" Zhang San ", 14] 
Object.entries({ name: ' Zhang San ', age:14 }) // (2) [Array(2), Array(2)]0: (2)["name", " Zhang San "]1: (2)["age", 14]

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