Benchmark: getElementsByClassName and querySelector

HTML Preparation code:

<div class="page-footer"></div>

Tests:

getElementsByClassName
const footerHeight = document.getElementsByClassName('page-footer')[0]
querySelector
const footerHeight = document.querySelector('.page-footer')[0]

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
getElementsByClassName
querySelector

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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**Benchmark Overview**

The benchmark measures the performance difference between two approaches: `getElementsByClassName` and `querySelector` for retrieving an element with a specific class name, in this case, `.page-footer`. The benchmark aims to identify which approach is faster on various devices and browsers.

**Options Compared**

The benchmark compares the following options:

1. **`getElementsByClassName`**: This method returns an HTMLCollection containing all elements that have the specified value of the `class` attribute. In this case, it's used to retrieve the element with class `page-footer`.
2. **`querySelector`**: This method returns the first element that matches the specified selector. The syntax is similar to CSS selectors, allowing for more flexibility and expressiveness.

**Pros and Cons**

*   **`getElementsByClassName`**:
    *   Pros: Simple and widely supported across browsers. It's also a non-standard method (not part of the ECMAScript specification), which can be beneficial for compatibility.
    *   Cons: This method returns an array-like object containing all matching elements, which can lead to slower performance compared to `querySelector`. Additionally, it may not return the expected result if there are multiple elements with the same class name but different tag names.

*   **`querySelector`**:
    *   Pros: Faster and more efficient than `getElementsByClassName`, especially for single-element matches. It's also a standard method (part of the DOM Level 2 specification) that provides better performance and accuracy.
    *   Cons: Less compatible across browsers, as it relies on CSS selector syntax.

**Library Used**

In this benchmark, no external library is explicitly mentioned. The tests only use built-in JavaScript methods (`getElementsByClassName` and `querySelector`) to retrieve the element.

**Special JS Feature or Syntax**

The `querySelector` method uses a CSS-like syntax for selectors, which allows for more flexibility in selecting elements based on their class names, IDs, tag names, attribute values, etc. The syntax used in this benchmark is:

```
const footerHeight = document.querySelector('.page-footer')[0];
```

This syntax is part of the CSS Selectors Level 1 specification.

**Other Alternatives**

If you need to retrieve an element by its class name but want to avoid using `getElementsByClassName`, you can consider using other methods like:

*   `document.querySelector`. This method returns a single element that matches the specified selector, which can be more efficient than `getElementsByClassName` for single-element matches.
*   `Element.prototype.matches()`. This is a modern method introduced in CSS Selectors Level 4, which provides better performance and accuracy compared to older selectors.

For example:

```
const footerHeight = document.querySelector('.page-footer')[0];
// or
const footerHeight = element.closest('.page-footer');
```

Related benchmarks:

getElementsByClassName and querySelector (version: 0)

Comparing performance of: getElementsByClassName vs querySelector

Created: 3 years ago by: Registered User

Jump to the latest result

getElementsByClassName

querySelector

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):