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

Overview

The benchmark compares two approaches to calculate the width of an HTML element: window.getComputedStyle and document.getElementById().getBoundingClientRect().width.

Test Cases

There are two test cases:

1. getComputedStyle: This test case uses a loop to repeatedly call checkDisplay('foo'), where checkDisplay function calculates the width of the HTML element with id "foo" using window.getComputedStyle.
2. getBoundingClientRect: This test case uses a similar approach as the first one, but instead of using window.getComputedStyle, it directly accesses the getBoundingClientRect() method on the document.getElementById('foo').

Library and Purpose

In both test cases, the checkDisplay function is not a built-in JavaScript library or syntax. It's a custom function created to measure the performance difference between these two approaches.

However, document.getElementById and window.getComputedStyle are built-in JavaScript APIs that interact with the DOM (Document Object Model).

Special JS Features/Syntax

There isn't any specific special JavaScript feature or syntax used in this benchmark. The code only relies on standard JavaScript functionality.

Pros and Cons of Approaches

Here's a brief summary of each approach:

1. getComputedStyle:
   • Pros: More accurate, as it takes into account the element's layout and box model.
   • Cons: Can be slower due to the need to fetch additional metadata from the browser.
2. getBoundingClientRect:
   • Pros: Faster, as it provides a more direct access to the element's size and position.
   • Cons: May not provide accurate results for elements with complex layouts or box models.

Other Alternatives

If you want to compare these two approaches in different scenarios, you could also consider using other methods:

1. Client-side CSSOM: Instead of getComputedStyle, you could use the Client-side CSS Object Model (CSSOM) API to access the element's styles.
2. Server-side rendering: You could measure the performance difference between these approaches when rendered on the server, rather than client-side.

Keep in mind that each approach has its own trade-offs and may not be suitable for all scenarios.