Super detailed summary of javascript array methods_javascript skills

1. Common methods of arrays
1: join();

Convert array to string for display. If no parameters are entered, they will be connected by commas by default; if parameters are entered, they will be connected by parameters.

var arr=[1,2,3];
console.log(arr.join());  // 1,2,3;
console.log(arr.join("_")); // 1_2_3;
console.log(arr);      // [1,2,3];

The original array remains unchanged.

2: reverse();

Arrange the array in reverse order and the original array is modified.

var arr=[1,2,3];
var arr2=arr.reverse();
console.log(arr2); // [3,2,1];
console.log(arr);  // [3,2,1];

3: sort();

By default, the array items are arranged in ascending order, call the toString() method of each array item, and then compare the resulting strings, starting from the beginning of the string.

var arr=[2,12,14,21,5];
console.log(arr.sort());  //[12, 14, 2, 21, 5];

You can also pass in a comparison function as a parameter. If the first parameter should come first, the comparison function returns a value less than 0; otherwise, the first parameter comes last, and the comparison function returns a value greater than 0; if the order does not matter, the comparison function returns 0;

var arr=[2,12,14,21,5];
console.log(arr.sort(function(a,b){return a-b}));   // [2,5,12,14,21]; 

var arr1=[2,12,14,21,5];
console.log(arr1.sort(function(a,b){return b-a}));  // [21,14,12,5,2];

4: concat();

Arrays are merged, and the original arrays remain unchanged.

var arr=[1,2,3];
console.log(arr.concat(4,5));      // [1, 2, 3, 4, 5];
console.log(arr.concat([4,5],6));    // [1, 2, 3, 4, 5, 6];
console.log(arr.concat([[4,5],6]));   // [1, 2, 3, [4, 5],6];
console.log(arr);            // [1, 2, 3];

5: slice();

Returns a partial array, including the array item corresponding to the first parameter, but not the array item corresponding to the second parameter. If the parameter passed in is less than 0, it will be counted from back to front, and the last item will be -1. If only one parameter is passed in, the returned array contains all elements from the starting position to the end of the array. The original array remains unchanged.

var arr=[1,2,3,4,5];
console.log(arr.slice(1,3));   // [2,3];
console.log(arr.slice(1));    // [2,3,4,5];
console.log(arr.slice(1,-1));  // [2,3,4];
console.log(arr);        // [1,2,3,4,5];

6: splice();

Array concatenation:

1). Delete - used to delete elements, two parameters, the first parameter (the position of the first item to be deleted), the second parameter (the number of items to be deleted);

2). Insertion-Insert any element into the specified position in the array. Three parameters, the first parameter (actual position), the second parameter (0), the third parameter (the inserted item);

3). Replacement - Insert any item element into the specified position of the array, and delete any number of items at the same time, three parameters. First parameter (starting position), second parameter (number of items to delete), third parameter (insert any number of items);

splice() returns an array consisting of deleted elements, or an empty array if no elements were deleted. The original array is modified.

var arr=[1,2,3,4,5,6];
console.log(arr.splice(2));  　　　　　 // [3,4,5,6];
console.log(arr);    　　　　　　　　 // [1,2];
console.log(arr.splice(2,0,3,4,5,6));  // [];
console.log(arr); 　　　　　　　　　　　　　 // [1,2,3,4,5,6];
console.log(arr.splice(2,2));　　　　　　　// [3,4];
console.log(arr);　　　　　　　　　　　　　　// [1,2,5,6];

7: push() and pop() methods, unshift() and shift() methods;

push() and pop() stack methods, last in first out. The original array is changed.

The push() method adds one or more elements to the end of the array and returns the new length of the array.

The pop() method removes the last element of the array, reduces the length of the array and returns the deleted value.

unshift() and shift() queue methods, first in first out. The original array is changed.

The unshift() method adds one or more elements to the head of the array, changes the index of the existing elements, and then returns the new length of the array.

The shift() method deletes the first element of the array and returns it, changing the index of the existing element.

var arr=[1,2,3];
console.log(arr.push(4));　　//4;
console.log(arr);　　　　　　 //[1,2,3,4];
console.log(arr.pop());　　　//4;
console.log(arr);　　　　　　 //[1,2,3];
console.log(arr.unshift(0)); //4;
console.log(arr);　　　　　　 //[0,1,2,3];
console.log(arr.shift());  //0;
console.log(arr);　　　　　　 //[1,2,3];

8：forEach();

forEach()里第一个参数为该集合里的元素，第二个参数为集合里的索引，第三个参数为集合本身。

9：map();

map()对数组中的每一项运行给定函数，返回每次函数调用的结果组成的数组，原数组未被修改。

10：every()

对数组的每一项运行给定函数，如果该函数对每一项都返回true，则返回true。

11：some()

对数组的每一项运行给定函数，如果该函数对任一项返回true，则返回true。

12：filter()

对数组中的每一项运行给定函数，返回该函数会返回true的项组成的数组。

13：reduce()和reduceRight();
两个方法都会迭代数组的所有项，然后构建一个最终返回的值。其中reduce()方法从数组的第一项开始，逐个遍历到最后。而reduceRight()则从数组的最后一项开始，向前遍历到第一项。数组未被修改。

二、扩展方法
1：数组去重

function unique(array){
  return array.filter(function(item,index){
    return array.indexOf(item)==index;
  })
};
var arr=[1,2,3,3,4,2,1,5];
console.log(unique(arr));　　　　//[1,2,3,4,5];

function unique(arr){
  var arr2=[arr[0]],
    len=arr.length;
  if(!len){
    return;
  }
  for(var i=0;i<len;i++){
    arr2.join(" ").indexOf(arr[i])<0&#63;arr2.push(arr[i]):"";
  }
  return arr2;
}
var arr=[1,2,3,3,4,2,1,5];
console.log(uniq(arr)); 　　//[1,2,3,4,5]    

2：去掉数组中的空元素

function deleteNullInArray(array){
  return array.filter(function(item){
    return item!=null;
  }) 
}
var arr=[1,2,null,,,5];
console.log(deleteNullInArray(arr));　　//[1,2,5];