Summary of array methods in Javascript (recommended)_javascript skills

Array.prototype defines many methods for operating arrays. Here are some methods in ECMAScript3

1.Array.join() method

This method converts the elements in the array into strings and connects them together according to the specified symbol, and returns the last generated string. It can contain a parameter, which is the symbol for connecting the array elements. The default is comma.

var ay = [1,2,3];
ay.join();       // =>"1,2,3" 
ay.join("+");     // => "1+2+3" 
ay.join(" ");     // =>"1 2 3"
ay.join("");      // =>"123"

var by = new Array(10) //新建一个长度为10的空数组
by.join("-");      //=> "---------" 连接10个空元素

2.Array.reverse() method

This method reverses the order of the elements in the array and returns the array in reverse order. This method will change the current array and will not create a new array.

3.Array.sort() method

This method sorts the elements in the array and returns the sorted array. When the sort() method takes no parameters, the array is sorted in alphabetical order. If the array contains undefined elements, they will be sorted to the end of the array.

We can also pass a comparison function as a parameter to the sort() method to sort the array using the specified comparison function. If the return value of the comparison function is less than 0, the first parameter comes first. On the contrary, if the return value is greater than 0, the second parameter comes first. If the two parameter values ​​are equal, 0
is returned.

This method creates and returns a new array, connecting the original array elements and each element in the method to form a new array. This method does not recursively call the parameters in the method.

This method returns a fragment or subarray of the specified array. This method can have two parameters, which specify the start and end positions of the fragment respectively. The returned array contains the element specified by the first parameter and all elements up to but not including the first The array element at the position specified by the two parameters. If there is only one parameter, it contains the specified start position to the end of the array. The parameter can be a negative value, indicating the position relative to the last element in the array. This method does not modify the array being called.

var e = [1,2,3,4,5,6];
e.splice(4);          //=> 返回[5,6] ; e是[1,2,3,4]
e.splice(1,2);         //=> 返回[2,3] ; e是[1,4]
      
var f = [1,2,3,4,5];
f.splice(2,0,"a","b");   //=>返回[]; f是[1,2,a,b,3,4,5]
f.splice(2,2,[6,7],3);   //=>返回[a,b]; f是[1,2,[6,7],3,4,5]

var a=[];     //[]
a.push(1,2);  //[1,2]
a.pop();      //[1]

a.unshift(2,3); //[2,3,1]
a.shift();      //[3,1]

var data=[1,2,3,4,5]
var sum = 0;
data.forEach(function(value){   //=>value为数组元素
  sum+=value;
})                        //=>15

data.forEach(function(value,i,a){ //=>三个参数分别指代数组元素，元素索引和数组
  a[i] = v+1;
})                        //=>data=[2,3,4,5,6]

var a=[1,2,3];
var b=a.map(function(v){
  return v*v;
})  //=> b=[1,4,9]

var a=[5,4,3,2,1]
var smalla=a.filter(function(v){
  return v<3; 
})                         //=>返回[2,1]
var everya=a.filter(function(v,i){ //=>i表示元素索引
  return i%2==0; 
})                         //=>返回[5,3,1]