JavaScript sets or returns the length property of the number of elements in an array

Definition and Usage

The length property sets or returns the number of elements in an array.

Syntax

arrayObject.length

Description

The length property of an array is always 1 greater than the subscript of the last element defined in the array. For regular arrays with consecutive elements starting with element 0, the length property declares the number of elements in the array.

The length property of an array is initialized when the array is created using Constructor Array(). When a new element is added to the array, the value of length is updated if necessary.

Set the length property to change the size of the array. If set to a value smaller than its current value, the array will be truncated and its trailing elements will be lost. If the set value is greater than its current value, the array will grow and new elements will be added to the end of the array, with their value being undefined.

Example

In this example, we will show how to use the length property to return and set the length of an array:

<script type="text/javascript">

var arr = new Array(3)
arr[0] = "John"
arr[1] = "Andy"
arr[2] = "Wendy"

document.write("Original length: " + arr.length)
document.write("<br />")

arr.length=5
document.write("New length: " + arr.length)

</script>

Output:

Original length: 3
New length: 5

About javascript Please see the following for a summary of the length attribute.

1. The length length property in

String

Object is the number of characters that returns the string.

For example:

// 普通字符串
var str = "abcdef";
console.log(str.length); // 6
// 数组
var str1 = new Array(1,2,3,4);
console.log(str1.length); // 4
// 数组与字符串
var str2 = str1 + str; // "abcdef1,2,3,4"
console.log(str2.length); // 13
// 对象和对象
var obj = {};
console.log(obj.length); // undefined
var obj += obj; // "[object Object][object Object]"
console.log(obj.length); // 30

2. Length in Function

Length can return the number of parameters of the function.

var a = function(a,b,c,d){};
console.log(a.length); // 4
var b = RegExp;
console.log(b.length); //new RegExp(pattern, attributes)构造方法中有两个参数, 所以length为2

※The length attribute of the arguments instance returns the actual number of parameters passed to the function by the calling program.

var a = function(){
  console.log(arguments.length); 
};
a(1,2,3); // 3
a(); // 0

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