Let's break down the provided benchmark and its various components.

Benchmark Definition JSON

The Benchmark Definition is a simple JavaScript expression that tests whether three different values are equal to a, which is initialized with the value 1. The expressions are:

1. if(a===undefined)console.log(a);
2. if(a===void 0)console.log(a);
3. if(a===u)console.log(a);

Here, void 0 and undefined are both used to represent the absence of a value, while u is a variable containing undefined. The === operator checks for strict equality between two values.

Options Compared

The three options being compared are:

1. undefined
2. void 0 (which is equivalent to undefined)
3. A variable u that contains the value undefined

These options represent different ways of checking whether a value is absent or null.

Pros and Cons of Each Approach

1. Undefined: The simplest way to check for an absent value, but it's not always clear what this means in the context of JavaScript. Some developers might interpret it as "not present" while others see it as "null-like behavior".
2. Void 0: This is a standard convention in JavaScript that indicates the absence of a value. However, some older browsers or versions of Internet Explorer (IE) may not support void 0. Additionally, modern browsers like Safari and Chrome might use undefined instead.
3. Variable containing undefined: Using a variable to represent an absent value can be confusing, especially for developers unfamiliar with JavaScript's nuances.

Library and Purpose

In this benchmark, there is no explicit library used. The expressions rely on built-in JavaScript features, such as the === operator and variables.

Special JS Feature or Syntax

None mentioned in the provided information.

Other Alternatives

While not explicitly mentioned in the benchmark, alternative approaches to checking for absent values could include:

• Using a specific check like if (a == null) console.log(a);
• Utilizing hasOwnProperty with an object, as in if (!u.hasOwnProperty('')) console.log(u);
• Employing more advanced techniques like using the Object.is() method or === with an explicit type coercion

However, these alternatives are not being tested or compared in this specific benchmark.

The provided benchmark offers a simple and straightforward way to compare how different browsers (or versions) handle absent values in JavaScript.