Javascript array length or size

JavaScript Array length

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

Syntax

Return the length of an array:

Set the length of an array:

Return Value

Related Pages:

Browser Support

length is an ECMAScript1 (ES1) feature.

ES1 (JavaScript 1997) is fully supported in all browsers:

Chrome	Edge	Firefox	Safari	Opera	IE
Yes	Yes	Yes	Yes	Yes	Yes

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Array.size() vs Array.length – JavaScript

In JavaScript, the “length” is a property of an array object that refers to the total number of array elements. On the other hand, “size()” is a method available in some programming languages for collections, such as lists, sets, and maps. It returns or outputs the number of elements present in the selected collection.

This article will describe the difference between size() and Array.length in JavaScript.

What is JavaScript Array.length Property?

“length” is the property of an array object. It is a read-only property of an array and can be used to determine the size or length of the array or to access the last element in the array. It can be accessed with the help of dot notation or bracket notation.

How to Use Array.length in JavaScript?

For using the length property of the array object to determine the size or length of an array, follow the given syntax:

Or use it with the bracket notation:

Example

Create an array of even numbers:

Determine the size of an array using the length property and store it in a variable “size”:

Lastly, print the length or size of the array:

What is size() in JavaScript?

“size()” is a JavaScript method utilized to determine or find out the size or an object’s length. It is also used for collections, such as “lists”, “sets”, and “maps”. However, it is not available for the array objects.

Example

Call the size() method with the defined array:

It gives an error “array.size is not a function” because the size() method is not available for the array:

That’s all about the array.size() and the array.length in JavaScript.

Conclusion

“size()” is a method available for collections like sets, lists, and maps. At the same time, “Array.length” is a property of an array object that represents the total number of elements in an array or the size/length of an array. However, the “length” property is significantly faster than a method call. This article described the difference between Array.size() and Array.length in JavaScript.

About the author

Farah Batool

I completed my master’s degree in computer science. I am an academic researcher and love to learn and write about new technologies. I am passionate about writing and sharing my experience with the world.

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Array: length

The length data property of an Array instance represents the number of elements in that array. The value is an unsigned, 32-bit integer that is always numerically greater than the highest index in the array.

Try it

Value

A nonnegative integer less than 2 32 .

Property attributes of Array: length
Writable	yes
Enumerable	no
Configurable	no

Description

The value of the length property is a nonnegative integer with a value less than 2 32 .

const listA = [1, 2, 3]; const listB = new Array(6); console.log(listA.length); // 3 console.log(listB.length); // 6 listB.length = 2 ** 32; // 4294967296 // RangeError: Invalid array length const listC = new Array(-100); // Negative numbers are not allowed // RangeError: Invalid array length 

The array object observes the length property, and automatically syncs the length value with the array’s content. This means:

• Setting length to a value smaller than the current length truncates the array — elements beyond the new length are deleted.
• Setting any array index (a nonnegative integer smaller than 2 32 ) beyond the current length extends the array — the length property is increased to reflect the new highest index.
• Setting length to an invalid value (e.g. a negative number or a non-integer) throws a RangeError exception.

When length is set to a bigger value than the current length, the array is extended by adding empty slots, not actual undefined values. Empty slots have some special interactions with array methods; see array methods and empty slots.

const arr = [1, 2]; console.log(arr); // [ 1, 2 ] arr.length = 5; // set array length to 5 while currently 2. console.log(arr); // [ 1, 2, ] arr.forEach((element) => console.log(element)); // 1 // 2 

Examples

Iterating over an array

In the following example, the array numbers is iterated through by looking at the length property. The value in each element is then doubled.

const numbers = [1, 2, 3, 4, 5]; const length = numbers.length; for (let i = 0; i  length; i++)  numbers[i] *= 2; > // numbers is now [2, 4, 6, 8, 10] 

Shortening an array

The following example shortens the array numbers to a length of 3 if the current length is greater than 3.

const numbers = [1, 2, 3, 4, 5]; if (numbers.length > 3)  numbers.length = 3; > console.log(numbers); // [1, 2, 3] console.log(numbers.length); // 3 console.log(numbers[3]); // undefined; the extra elements are deleted 

Create empty array of fixed length

Setting length to a value greater than the current length creates a sparse array.

const numbers = []; numbers.length = 3; console.log(numbers); // [empty x 3] 

Array with non-writable length

The length property is automatically updated by the array when elements are added beyond the current length. If the length property is made non-writable, the array will not be able to update it. This causes an error in strict mode.

"use strict"; const numbers = [1, 2, 3, 4, 5]; Object.defineProperty(numbers, "length",  writable: false >); numbers[5] = 6; // TypeError: Cannot assign to read only property 'length' of object '[object Array]' numbers.push(5); // // TypeError: Cannot assign to read only property 'length' of object '[object Array]' 

Specifications

Browser compatibility

BCD tables only load in the browser

See also

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This page was last modified on Jun 27, 2023 by MDN contributors.

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