I'd be happy to explain the benchmark being tested on MeasureThat.net.

Benchmark Definition

The benchmark is testing two different approaches for checking if a variable a is undefined:

1. Using typeof a === 'undefined'
2. Using a === 'undefined'

Let's break down each approach:

Approach 1: typeof a === 'undefined'

This approach uses the typeof operator to check the type of a. In JavaScript, typeof returns one of six values:

• "string"
• "number"
• "boolean"
• "object"
• "function"
• "undefined"

By comparing a with 'undefined', we're essentially checking if a is undefined.

Pros:

• This approach is concise and easy to read.
• It's a commonly used pattern in JavaScript.

Cons:

• The comparison can be slower than other approaches, since it involves creating a new string object.
• Some modern browsers may optimize this comparison away in certain situations.

Approach 2: a === 'undefined'

This approach uses a simple equality check between a and the string 'undefined'.

Pros:

• This approach is often faster than Approach 1, since it only involves a single comparison.
• It's less prone to browser optimizations, which can affect the results.

Cons:

• The code is slightly more verbose than Approach 1.
• Some older browsers may not support this comparison correctly.

Library Used

Neither of these approaches relies on any external libraries. However, if we were to use a library for benchmarking or performance testing, some popular options include:

• V8 Benchmark
• Google's Performance Benchmark
• WebKit Benchmarks

Special JS Feature/Syntax

There are no special JavaScript features or syntax used in this benchmark that would require explanation.

Other Alternatives

If you were to rewrite this benchmark using modern JavaScript features, here are some alternatives to consider:

• Using === and adding a non-standard [[Prototype]] access property: a === { [[Prototype]]: {} }
• Using the is-undefined function from the lodash.is undefined library
• Using a more concise and expressive approach with destructuring: const { isUndefined } = require('lodash.is undefined'); if (!isUndefined(a))

However, these alternatives are not directly related to the benchmark being tested on MeasureThat.net.

In summary, this benchmark is testing two different approaches for checking if a variable is undefined in JavaScript. Approach 1 uses typeof and has some performance-related considerations, while Approach 2 uses simple equality checks and is generally faster but slightly more verbose.