Let's break down the provided benchmark definition and test cases to understand what is being tested.

Benchmark Definition: The benchmark is testing two approaches for removing an item from an array of objects by property:

1. Using findIndex and splice: This approach involves finding the index of the item in the array using the findIndex method, which returns the lowest index at which this value can be found in the array. Once the index is found, the splice method is used to remove the item from the array.
2. Using filter and reassignment: This approach involves creating a new array that includes only the items that do not match the specified condition using the filter method. The resulting filtered array is then reassigned back to the original variable.

Options Compared: The benchmark is comparing the performance of these two approaches:

• findItem & splice: Uses findIndex and splice
• reassignement & filter: Uses filter and reassignment

Pros and Cons:

• FindItem & Splice:
  • Pros:
    • Can be more efficient for large arrays, as it only requires finding the index of the item once.
    • Can be less memory-intensive, as it only creates a new array with the filtered items.
  • Cons:
    • Requires calling findIndex and splice, which can be slower than a single method call.
    • May require more code and is more prone to errors due to the additional step of finding the index.
• Reassignement & Filter:
  • Pros:
    • Can be simpler to implement, as it only requires creating a new array using filter.
    • Can be less error-prone, as it only involves a single method call.
  • Cons:
    • May require more memory, as it creates a new array with the filtered items.
    • Can be slower for large arrays, as it requires iterating over all items in the original array.

Library/Functionality Used: None of the provided benchmark test cases use any external libraries. However, they do utilize JavaScript's built-in methods:

• findIndex: Returns the lowest index at which this value can be found in the array.
• splice: Removes the item at the specified index from the array.
• filter: Creates a new array that includes only the items that pass the test.

Special JS Feature/Syntax: The benchmark uses a feature of JavaScript called "Arrow Functions" (o => o.a === 'c'), which is used as the callback function in the findIndex and filter methods. Arrow functions are a concise way to define small, single-expression functions. They were introduced in ECMAScript 2015 (ES6) and provide a more concise syntax for defining functions.

Alternatives: Other alternatives for removing an item from an array of objects by property could include:

• Using map and then slicing the resulting array
• Using forEach and pushing or splicing the result into a new array
• Using a library like Lodash, which provides a filterBy method for filtering arrays

It's worth noting that the best approach will depend on the specific use case and performance requirements.