length

Summary

Basically specifies the number of elements (AKA length) in an Array object. This means the length property represents a number one greater than the largest index defined in an Array object.

Syntax

arrayObj.length;
arrayObj.length = numVar;

arrayObj: Required. Any Array object.
numVar: Required. Any number less than 2 to the 32nd power (2^32).

Return Value

Any number less than 2 to the 32nd power (2^32). The type of its value must be the unsigned 32 bit integer in the range 0 through 4294967295, inclusive.

Examples

The array classes represents the content list of some training course. They are all displayed in the console by iterating over the array classes.



var classes = ["HTML", "CSS", "JavaScript"];
document.write("This course includes:<br>");
for (var i = 0, j = classes.length; i < j; i++) {
    document.write(classes[i] + "<br>");
}
// Output:
// This course includes:
// HTML
// CSS
// JavaScript

[View live example](http://code.webplatform.org/gist/9086506)

This shows how to limit the number of items added into a shopping cart. The length property shortens the array cart to a length of 3 if the current length is larger than 3. The attributes of the length property are { Writable: true, Enumerable: false, Configurable: false }. So you can set the length property to extend or truncate an Array object at any time.

var cart = ["bread", "cheese", "coffee"];

cart.push("salad"); // length = 4
if (cart.length > 3) {
    alert("Don't add more than 3 items!");
    cart.length = 3;
    document.write(cart);
}

// Output: bread,cheese,coffee

View live example

Remarks

In JavaScript arrays are sparse, and the elements in an array do not have to be contiguous. The length property is not necessarily the number of elements in the array. For example, in the following array definition, my_array.length contains 7, not 2:

var my_array = new Array( );
my_array[0] = "Test";
my_array[6] = "Another Test";
console.log(my_array.length);

// Output
// 7

Even if you set length to a number greater than its previous value, the number of actual elements does not increase.

console.log(my_array.length); // 7
my_array.length = 10;
console.log(Object.keys(my_array));

// Output
// ["0", "6"]

On the other hand, when decreasing length, the array is truncated.

console.log(my_array.length); // 10
my_array.length = 1;
console.log(Object.keys(my_array));

// Output
// ["0"]

Attributions

Microsoft Developer Network: Article