Benchmark: Deep querySelector scope vs no scope

HTML Preparation code:

<div id='parent'>
  <div class="foo">
    <div id="bar" class="bar">
      <div class="baz">
      </div>
    </div>
  </div>
</div>

Tests:

querySelector scope
document.getElementById("bar").querySelector(':scope > .baz')
querySelector no scope
document.querySelector('.baz')

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
querySelector scope
querySelector no scope

Fastest: N/A

Slowest: N/A

Latest run results:

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

LLMs can make mistakes. Check important info.

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

**Benchmark Overview**

The test is measuring the performance difference between using `querySelector` with the `:scope>` pseudo-class versus not using it at all, in both JavaScript engines (likely V8 for Chrome/Node.js and SpiderMonkey for Firefox).

**`:scope>` Pseudo-Class**

The `:scope` pseudo-class was introduced in ECMAScript 2019 (ES11) as a way to limit the scope of an element's styles or properties. When used with `querySelector`, it allows selecting elements within a specific scope, which is typically the document's DOM tree.

In this benchmark, the test case uses `document.getElementById("bar").querySelector(':scope > .baz')` to query for all elements directly descended from `#bar`. This effectively limits the search space to only elements within the `<div id="bar">` element. Without the `:scope>` pseudo-class, the query would need to traverse the entire DOM tree from `#bar`, which is likely slower.

**Options Compared**

The benchmark compares two approaches:

1. **Using`:scope`**: This approach uses the `:scope>` pseudo-class to limit the search space to elements within a specific scope.
2. **Not using`:scope`**: This approach performs a full DOM traversal from `#bar` without any scope limitation.

**Pros and Cons**

* Using`:scope`:
	+ Pros: Can significantly improve performance by limiting the search space.
	+ Cons: May not work as expected if the DOM structure is complex or has many nested elements.
* Not using`:scope`:
	+ Pros: Works consistently across all browsers and DOM structures.
	+ Cons: May be slower due to full DOM traversal.

**Library Usage**

There are no libraries mentioned in this benchmark, so we can assume that the JavaScript engines (V8 and SpiderMonkey) will handle the execution of these queries internally.

**Special JS Features or Syntax**

The benchmark uses a feature introduced in ECMAScript 2019 (ES11), specifically the `:scope>` pseudo-class. This is a relatively new feature, but it's becoming more widely supported across browsers.

**Other Alternatives**

If you wanted to write this benchmark from scratch, you could use a library like `benchmark` or `micro-benchmark` to implement the test cases. You would need to write a custom script that parses the HTML, selects the elements using both approaches, and measures their execution time.

Here's an example of how you might write this benchmark in Node.js using `benchmark`:
```javascript
const Benchmark = require('benchmark');
const jsdom = require('jsdom');

// Create a DOM parser and renderer
const { JSDOM } = jsdom;
const dom = new JSDOM('<html><body></body></html>');

// Define the test cases
const testCases = [
  {
    name: 'querySelector scope',
    fn: (context) => {
      const barElement = dom.window.document.getElementById('bar');
      return barElement.querySelector(':scope > .baz');
    },
  },
  {
    name: 'querySelector no scope',
    fn: (context) => {
      const barElement = dom.window.document.getElementById('bar');
      return dom.window.document.querySelector('.baz');
    },
  },
];

// Run the benchmark
const suite = new Benchmark.Suite;
testCases.forEach((testCase) => {
  suite.add(testCase.name, testCase.fn);
});
suite.on('complete', () => {
  console.log(suite.filter('fastest').map((entry) => entry.name));
});
suite.run();
```
Note that this is just an example and might require modifications to fit your specific use case.

Related benchmarks:

Deep querySelector scope vs no scope (version: 0)

Comparing performance of: querySelector scope vs querySelector no scope

Created: 2 years ago by: Guest

Jump to the latest result

querySelector scope

querySelector no scope

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):