How to get the length of an array in js

Getting the length of an array in JS is very simple. Each array has a length attribute, which returns the maximum length of the array, that is, its value is equal to the maximum subscript value plus 1. Since the numeric subscript must be less than 2^32-1, the maximum value of the length attribute is equal to 2^32-1.

Example 1

The following code defines an empty array, and then assigns a value to the element with the subscript equal to 100, then the length attribute returns 101. Therefore, the length attribute cannot reflect the actual number of array elements.

var a = [];  //声明空数组
a[100] = 2;
console.log(a.length);  //返回101

length property is readable and writable and is a dynamic property. The length attribute value will also be automatically updated as the array elements change. At the same time, if the length attribute value is reset, it will also affect the elements of the array. The details are as follows: If the length attribute is set to a value smaller than the current length value, the array will be truncated and the element values ​​beyond the new length will be lost. . If the length property is set to a value greater than the current length value, then the empty array will be added to the end of the array, causing the array to grow to the newly specified length, and all read values ​​will be undefined.

Example 2

The following code demonstrates the impact of dynamic changes in the length attribute value on the array.

var a = [1,2,3];  //声明数组直接量
a.length = 5;  //增长数组长度
console.log(a[4]);  //返回undefined，说明该元素还没有被赋值
a.length = 2;  //缩短数组长度
console.log(a[2]);  //返回undefined，说明该元素的值已经丢失

The above is the detailed content of How to get the length of an array in js. For more information, please follow other related articles on the PHP Chinese website!