Measuring the performance of JavaScript microbenchmarks can be fascinating, and it's great that you're interested in understanding what's being tested.

What is being tested?

The provided benchmark tests two different approaches for inserting an element into an array:

1. splice() insertion: This method modifies the original array by removing the specified number of elements from the beginning (in this case, 0) and then inserts a new element at that position.
2. Spread insertion (...arr, 4): This method creates a new array by spreading the existing array and then appending the new element.

Options comparison

The two approaches have different pros and cons:

• Splice() insertion: This approach is more efficient when inserting elements at the beginning of an array because it modifies the original array. However, it can be slower for large arrays because it requires shifting all subsequent elements to fill the gap.
• Spread insertion (...arr, 4): This approach creates a new array without modifying the original array. It is more efficient for large arrays and avoids potential performance issues with shifting elements.

Library usage

In this benchmark, there doesn't appear to be any specific JavaScript library being used. However, some browsers like Opera might have their own implementation of certain functions (e.g., Array.prototype.splice() or spread syntax) that might affect the results.

Special JS features or syntax

The benchmark does not use any special JavaScript features or syntax beyond what's standard in modern browsers.

Other alternatives

Alternative approaches to inserting elements into an array could include:

• Using concat(), which also creates a new array but can be slower than spread insertion for large arrays.
• Using Array.prototype.push() and then using shift() to remove the element, although this is less efficient than both splice insertion and spread insertion.

Here's some sample code that demonstrates these alternatives:

// Splice() insertion
let arr = [1, 2, 3];
arr.splice(0, 0, 4); // More efficient for small arrays

// Spread insertion
let newArr = [...arr, 4]; // Creates a new array without modifying the original

// Concat()
let arr2 = [1, 2, 3];
let newArr2 = arr2.concat([4]); // Creates a new array but can be slower than spread insertion for large arrays

// Push() and shift()
let arr3 = [1, 2, 3];
arr3.push(4);
while (arr3.length > 0) {
    arr3.shift(); // Less efficient than both splice insertion and spread insertion
}