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

Overview

The benchmark compares the performance of four different loop constructs:

1. Traditional for loop
2. forEach method (a part of the Array prototype)
3. some method (a part of the Array prototype)
4. for...of loop (introduced in ECMAScript 2015)

Loop Constructs

Let's examine each loop construct and its characteristics:

1. Traditional for loop: This is a basic loop construct that uses a variable (i) to iterate over an array. The loop continues as long as the condition (--i) is true.
2. forEach method: This is a part of the Array prototype that allows you to iterate over an array without using a traditional for loop. It takes two arguments: the callback function and the current element in the iteration. In this benchmark, the callback function simply accesses the current element (array[i];).
3. some method: This is another part of the Array prototype that returns true if at least one element in the array passes a test provided by a function. In this benchmark, the callback function also accesses the current element (array[i];), but its purpose is to check if any element satisfies a condition.
4. for...of loop: This is a new type of loop introduced in ECMAScript 2015 that allows you to iterate over arrays and other iterable objects without using traditional for loops or array methods like forEach.

Pros and Cons

Here are some pros and cons of each loop construct:

1. Traditional for loop: Pros: simple, well-established, and widely supported. Cons: can be verbose and error-prone.
2. forEach method: Pros: concise, easy to read, and convenient for array iteration. Cons: may not be as efficient as traditional loops for large arrays.
3. some method: Pros: concise and expressive, suitable for simple array iteration with a condition. Cons: can lead to performance issues if used incorrectly or with very large arrays.
4. for...of loop: Pros: modern, concise, and efficient for array iteration. Cons: may not be supported by older browsers or environments.

Library Usage

In this benchmark, the forEach, some, and no library is used by default. The array variable is created using new Array(100).fill(1);, which initializes an array with 100 elements filled with the value 1.

Special JS Features/Syntax

The benchmark doesn't use any special JavaScript features or syntax that are not part of the standard language. However, it does rely on the ECMAScript 2015 for...of loop, which is a relatively new feature.

Alternatives

Other alternatives for iterating over arrays include:

1. Using traditional loops with indices (i) and array length checks.
2. Using other array methods like map, filter, or reduce.
3. Utilizing modern libraries like Lodash or Ramda for functional programming.
4. Leveraging new languages or frameworks that support advanced iteration constructs.

I hope this explanation helps!