Benchmark: getElementsByClassName vs querySelector, performance

HTML Preparation code:

<div class="testClass"></div>

Tests:

getElementByClassName
var el = document.getElementsByClassName('testClass')[0]; var className = el.className;
querySelector
var el = document.querySelector('.testClass'); var className = el.className;

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
getElementByClassName
querySelector

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: 2 years ago)

User agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) QtWebEngine/5.15.2 Chrome/87.0.4280.144 Safari/537.36

Browser/OS: Chrome 87 on Linux

View result in a separate tab

Test name	Executions per second
getElementByClassName	3018389.2 Ops/sec
querySelector	3386053.2 Ops/sec

Autogenerated LLM Summary (model llama3.2:3b, generated 2 months ago):

LLMs can make mistakes. Check important info.

Let's break down what's being tested in this benchmark.

**Benchmark Overview**

The benchmark compares the performance of two approaches to retrieve an HTML element by its class name:

1. `document.getElementsByClassName('testClass')[0]`
2. `document.querySelector('.testClass')`

**Library and Purpose**

In both test cases, a library (BOM: Browser Object Model) is being used, which provides a set of APIs to interact with the web browser.

The specific libraries used are:

* `document`: an object that represents the document element in the HTML document.
* `getElementsByClassName` and `querySelector`: methods of the `document` object that allow you to retrieve elements by their class name or selector, respectively.

**Options Compared**

Two options are being compared:

1. **`getElementsByClassName`**: This method returns a live HTMLCollection object containing all elements with the specified class name(s). It's an older API that was widely supported in older browsers.
2. **`querySelector`**: This method returns the first element that matches the specified selector. It's a more modern and flexible API that allows for more powerful selectors.

**Pros and Cons**

Here are some pros and cons of each approach:

* `getElementsByClassName`:
	+ Pros: widely supported, relatively simple to use.
	+ Cons: can be slower than `querySelector`, as it needs to iterate over all elements with the class name(s).
* `querySelector`:
	+ Pros: faster and more efficient, allows for more powerful selectors.
	+ Cons: may not be supported in older browsers.

**Special JS Feature or Syntax**

Neither of these test cases uses any special JavaScript features or syntax. They are simple, straightforward examples that illustrate the performance difference between two DOM manipulation methods.

**Other Alternatives**

Other alternatives to `getElementsByClassName` and `querySelector` include:

* Using CSS selectors with `document.querySelector`: `document.querySelector('.testClass')`
* Using a library like jQuery: `$('.testClass')[0]`

However, these alternative approaches are not being tested in this benchmark.

I hope this explanation helps!

Related benchmarks:

getElementsByClassName vs querySelector, performance (version: 0)

Comparing performance of: getElementByClassName vs querySelector

Created: 3 years ago by: Guest

Jump to the latest result

getElementByClassName

querySelector

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated 2 months ago):