Benchmark: querySelector vs getElementsByClassName[0]

HTML Preparation code:

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

Tests:

querySelector
document.querySelector(".test")
getElementsByClassName
document.getElementsByClassName(".test")[0]

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
querySelector
getElementsByClassName

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: one year ago)

User agent: Mozilla/5.0 (Android 13; Mobile; rv:109.0) Gecko/110.0 Firefox/110.0

Browser/OS: Firefox Mobile 110 on Android

View result in a separate tab

Test name	Executions per second
querySelector	1034386.3 Ops/sec
getElementsByClassName	1562036.4 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 definition and test cases to understand what's being tested.

**Benchmark Definition**

The benchmark definition is a JSON object that contains information about the benchmark, including:

* `Name`: A unique name for the benchmark.
* `Description`: A brief description of the benchmark. In this case, it's a slight alteration because `querySelector` only returns the first element when used with a class selector.
* `Script Preparation Code`: This field is empty in this example, which means no script is executed before running the test.
* `Html Preparation Code`: This field contains HTML code that is used to prepare the test environment. In this case, it creates two `<div>` elements with the same class name (`test`) and returns an array of results.

**Individual Test Cases**

The benchmark consists of two individual test cases:

1. **querySelector**
	* `Benchmark Definition`: The script to execute: `document.querySelector(".test")`.
	* This test case is designed to measure the performance of using the `querySelector` method to select a single element by class name.
2. **getElementsByClassName**
	* `Benchmark Definition`: The script to execute: `document.getElementsByClassName("test")[0]`.
	* This test case is designed to measure the performance of using the `getElementsByClassName` method to select an array of elements and then returning only the first one.

**Library Usage**

In both test cases, no external library is used. However, it's worth noting that if a library like jQuery were used, its methods would be tested instead.

**Special JS Feature or Syntax**

There are no special JavaScript features or syntax mentioned in this benchmark.

**Pros and Cons of Different Approaches**

The two approaches compared here are:

* `querySelector`: Returns the first matching element. This approach is often preferred because it's more concise and efficient.
* `getElementsByClassName`: Returns an array of all elements that match the class name, and then returns only the first one (by indexing `[0]`). This approach can be slower than `querySelector` for single-element matches.

**Pros of `querySelector`**

* More concise
* Typically faster
* Widely supported

**Cons of `querySelector`**

* Can be less intuitive to use, especially for complex queries
* May not work as expected in older browsers or with certain CSS properties

**Pros of `getElementsByClassName`**

* Can be more flexible when dealing with multiple elements that match the class name
* Works consistently across all browsers and platforms

**Cons of `getElementsByClassName`**

* Typically slower than `querySelector`
* Requires additional code to extract only the first element from the array

**Other Alternatives**

If you need to select elements based on other attributes or conditions, other approaches might be more suitable:

* Using `querySelectorAll` with a CSS selector that includes an attribute (e.g., `[data-id]`) instead of just the class name.
* Using a library like jQuery's `.closest()` method for more complex element selection.

Keep in mind that these alternatives are not directly related to the specific trade-offs between `querySelector` and `getElementsByClassName`.

Related benchmarks:

querySelector vs getElementsByClassName[0] (version: 0)

Slight alteration because querySelector only returns a first element.

Comparing performance of: querySelector vs getElementsByClassName

Created: 6 years ago by: Guest

Jump to the latest result

querySelector

getElementsByClassName

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):