Benchmark: querySelector vs getElementsByClassName[0] Perf

HTML Preparation code:

<div class="test"></div>

Tests:

QuerySelector
document.querySelector(".test");
GetElementsByClassName[0]
document.getElementsByClassName("test")[0];

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
QuerySelector
GetElementsByClassName[0]

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: 5 years ago)

User agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36

Browser/OS: Chrome 70 on Windows 7

View result in a separate tab

Test name	Executions per second
QuerySelector	1465858.6 Ops/sec
GetElementsByClassName[0]	1418027.6 Ops/sec

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

LLMs can make mistakes. Check important info.

Let's break down the provided benchmark and explain what's being tested.

**Benchmark Overview**

The benchmark measures the performance of two approaches to select an HTML element: `document.querySelector` and `document.getElementsByClassName`. The goal is to determine which approach is faster for retrieving a single element by its class name or CSS selector.

**Options Compared**

Two options are compared:

1. **`document.querySelector`**: This method uses CSS selectors to find the first matching element in the document.
2. **`document.getElementsByClassName`**: This method returns an array of all elements with the specified class name, and we're interested in the first (and only) element.

**Pros and Cons**

*   `document.querySelector`:
    *   Pros: flexible and powerful for selecting multiple elements using CSS selectors, less code required.
    *   Cons: slower than `getElementsByClassName[0]` for single-element retrieval, requires more CPU cycles to parse the selector.
*   `document.getElementsByClassName`:
    *   Pros: faster for single-element retrieval, uses less CPU cycles since it only returns one element.
    *   Cons: less flexible and powerful compared to `querySelector`, requires explicit indexing to retrieve individual elements.

**Library/Functionality**

Neither of these functions relies on a library. They are native JavaScript methods built into the DOM API.

However, note that the specific behavior might be affected by CSS-specific features like `class` attributes or pseudo-classes like `.test: nth-child(1)` in this benchmark.

The alternative for retrieving an element with a given class name would be using a library such as jQuery. It has its own set of methods to retrieve elements, including `$`.

Related benchmarks:

querySelector vs getElementsByClassName[0] Perf (version: 0)

Comparing performance of: QuerySelector vs GetElementsByClassName[0]

Created: 6 years ago by: Guest

Jump to the latest result

QuerySelector

GetElementsByClassName[0]

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):