Benchmark: querySelector vs getElementsByClassName get first element

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:

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

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Let's break down the provided benchmark and explain what's being tested, compared, and what pros/cons of each approach are.

**Benchmark Overview**

The benchmark measures the performance difference between two methods for selecting an element in an HTML document: `document.querySelector()` and `document.getElementsByClassName()`. The test is designed to get the first element that matches a specific class (`test`).

**Script Preparation Code**

There's no script preparation code provided, which means that the JavaScript environment is assumed to be empty. This simplifies the benchmarking process by allowing only the performance difference between the two methods to be measured.

**Html Preparation Code**

The HTML preparation code consists of two identical `<div>` elements with the class `test`. This setup ensures that both methods will select the same element, making it a fair comparison.

**Individual Test Cases**

There are two test cases:

1. **querySelector**: Measures the performance of using `document.querySelector()` to get the first element with the class `test`.
2. **getElementsByClassName**: Measures the performance of using `document.getElementsByClassName()` to get an array of elements with the class `test`, and then selecting the first element from that array.

**Comparison**

The comparison between these two methods is based on which one performs better in terms of execution speed. The benchmark measures how many executions (i.e., invocations) each method can perform per second.

**Pros and Cons**

1. **querySelector**
	* Pros:
		+ More efficient, as it only returns the first matching element, reducing unnecessary processing.
		+ Fewer DOM mutations, which can lead to better performance.
	* Cons:
		+ May require more CPU cycles due to its more complex parsing logic.
2. **getElementsByClassName**
	* Pros:
		+ Generally faster for large datasets, as it can return an array of matching elements at once.
	* Cons:
		+ More DOM mutations occur when accessing the first element from the array (e.g., `.item(0)`), which can lead to performance issues.

**Library and Syntax Considerations**

None mentioned in this benchmark. However, if you were to extend this benchmark, you might consider exploring other libraries or syntax features that affect performance, such as:

* Using `QuerySelectorAll()` for more complex selectors
* Utilizing modern JavaScript features like `Array.prototype.filter()` or `String.prototype.includes()`
* Implementing custom parsing logic using regular expressions

**Other Alternatives**

If you wanted to expand this benchmark, some alternative methods could be considered:

* **Document.querySelectorAll()**: This method is similar to `querySelectorAll()`, but with an array of matching elements as its return value.
* **CSS Selectors**: Using CSS selectors like `:first-child` or `> .test` can provide more efficient and flexible querying options.
* **MutationObserver API**: This API allows you to observe changes to the DOM and react accordingly, which might be more suitable for certain use cases.

Keep in mind that each alternative has its own strengths and weaknesses, and their performance characteristics may vary depending on the specific use case.

Related benchmarks:

querySelector vs getElementsByClassName get first element (version: 0)

Comparing performance of: querySelector vs getElementsByClassName

Created: 7 years ago by: Guest

Jump to the latest result

querySelector

getElementsByClassName

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

No previous run results

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