How to define an array in js? Summary of the definition and operation examples of arrays in js

What are the ways to define arrays and array operations in js? There are two ways to define arrays in js: defining arrays literally and using the new keyword to define arrays. The operations of arrays in js include adding array elements, deleting array elements, modifying array elements, and traversing array elements. Let’s take a closer look at these. content.

1. The definition of js array

1. The literal definition of the array is as follows:

<script>
    var arr = (1,2,3,[4,5,6],{}); //其中数组元素可以是number，string，object
    console.log(arr);//在控制台输出数组</script>

2. Use the new keyword to define the array as follows:

<script>var arr = new Array[1,2,3,[4,5,6],{}];
console.log(arr);

3. Note:

When using new to define an array, if it is defined as: var arr = new Array(n); it means that an array with an array length of n is defined. If A null value means an empty array is defined.

2. js array operations

1. Add array elements as follows:

Method 1:

<script>
    var arr = [1,2,3,4,"A"];
    arr[5] = "M"; //如果为数组元素赋值的索引i大于数组长度，则添加数组元素后的数组长度为 i+1
    console.log(arr);</script>

Method 2: Use push() method;

<script>
    var arr = [1,2,3,4,"A"];
    arr.push("BB","cc");//在数组元素的后面添加，可以追加多个元素，
    console.log(arr);</script>

Method 3: Use unshift() method;

<script>
    var arr = [1,2,3,4,"A"];
    arr.unshift("BB","cc");//在数组元素的前面添加，可以追加多个元素，
    console.log(arr);</script>

2. Delete array elements:

Method 1: Use pop() method, Delete the last array element;

<script>
    var arr = ["A", "B", "C", "D"];
    arr.pop(); //删除最后一个元素，没有参数，每次只能删除一个
    console.log(arr);</script>

Method 2: Use the shift() method to delete the first element

<script>
    var arr = ["A", "B", "C", "D"];
    arr.shift(); //删除第一个数组元素，没有参数，每次只能删除一个
    console.log(arr); </script>

3. Modify the array

 <script>
    var arr = ["A", "B", "C", "D"];
    arr[2] = "E";
    console.log(arr); </script>

4. Traverse one dimension Array

<script>
    var arr = ["A", "B", "C", "D"];
    console.log(arr.length);    for(var i = 0; i<arr.length;i++){
        console.log(arr[i]);
    }</script>

5. Traversing a two-dimensional array

   <script>
        var arr = [
            [1, 2, 3],
            [4, 5, 6],
            [7, 8, 9]
        ]        for(var i = 0;i<arr.length;i++){ //先遍历arr
            for(var j = 0;j<arr[i].length;j++){ //遍历arr元素中的数组元素
                console.log(arr[i][j]);
            }
        }    </script>

3. Common methods in js arrays

1. join(): add the The elements are connected using a certain symbol to form a string

 <script>    
 var arr = ["A","B","C","D"]    var a = arr.join("*")    console.log( a);    
 var a = arr.join("+")    console.log( a);    var a = arr.join("-")    console.log( a);    
 var a = arr.join("/")    console.log(a);    console.log(typeof a);
</script>

2. reverse(): Reverse the order of the elements in the array;

  <script>
      var arr = ["A","B","C","D"]      
      var a = arr.reverse();  //倒置数组顺序
      for(var i = 0; i< arr.length; i++) //遍历数组
      {
        console.log(a[i]);
      }      
    </script>

3. concat(): Convert multiple Arrays are spliced ​​into one array, and the multi-dimensional array will not be expanded

 <script>
     var arr = ["A","B","C","D"];     
     var arr1 = ["A","B","C","D"];     
     var arr2 = ["A","B","C","D"];     
     var arr3 = ["A","B","C","D" ,[1,2,3]];     
     var a = arr.concat(arr1,arr2,arr3) //concat将多个数组拼接成一个数组,其中的多维数组是不会展开的 
     console.log(a);
     console.log(typeof a)  
    </script>

4. sort(): Sort the elements in the array from small to large according to ASCII

 <script>
       var arr = ["d","AbC","AaC","D",1,8,6];       
       var a = arr.sort(); //sort是单个字符比较 例如AbD和AaC 先比较第一个字母，在比较第二个字母，第二个字母那个小就排在前面，之后就不比较后面的字母大小了
       console.log(a);    </script>

5. slice (): Intercept elements in the array;

   <script>
         var arr = ["A","B","C","D"];         
         var a = arr.slice(1,3); //截取的元素包括开始截取的元素，但是不包括最后截取的元素
         console.log(a);         
         var a = arr.slice(1); //只写一个则表示截取到最后一个元素
         console.log(a);    </script>

6. toString(): Convert the array into a string type

<script>
      var arr = ["A","B","C","D"];      
      var a = arr.toString();
      console.log(a);
      console.log(typeof a);    
 </script>

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