Let's break down the benchmark and its test cases.

Benchmark Definition:

The benchmark compares three approaches to append elements to an array of size 100,000:

1. Using push() method (Control)
2. Using concat array method (Concat)
3. Using spread operator (...) to add elements to an existing array (Spread)

Script Preparation Code:

window.top.tests = {control:[], concat:[], spread:[]};
window.test = (new Array(100000)).fill(null);
window.cutoff = 99000;

This code sets up three arrays on the window.top object: control, concat, and spread. It also creates an array of size 100,000 filled with null values and assigns it to window.test.

Html Preparation Code: (empty string)

Since there is no HTML preparation code provided, we can assume that the benchmark only tests JavaScript performance.

Individual Test Cases:

Each test case measures the execution time for a specific approach:

1. Control (for push):

for (let element of window.test) window.top.tests.control.push(element);
if (window.top.tests.control.length > window.cutoff) { window.top.tests.control = []; console.log('reset control'); }

This test case uses the push() method to append elements to the control array. It loops through the entire window.test array and pushes each element onto the control array. When the length of the control array exceeds a certain threshold (cutoff), it resets the array.

2. Concat:

window.top.tests.concat = window.top.tests.concat.concat(window.test);
if (window.top.tests.concat.length > window.cutoff) { window.top.tests.concat = []; console.log('reset concat'); }

This test case uses the concat() method to append the entire window.test array to the concat array. It does this by assigning a new array created from the concatenation of the existing concat array and the window.test array back to the concat property.

3. Spread:

window.top.tests.spread.push(...window.test);
if (window.top.tests.spread.length > window.cutoff) { window.top.tests.spread = []; console.log('reset spread'); }

This test case uses the spread operator (...) to add all elements from the window.test array to the spread array. The resulting array is then pushed onto the spread array using the push() method.

Pros and Cons of each approach:

• Control (for push):
  • Pros: Simple, efficient way to append elements.
  • Cons: Can lead to frequent resizing of the underlying array when appending large numbers of elements.
• Concat:
  • Pros: Allows for more control over array construction, as it creates a new array each time.
  • Cons: Creates unnecessary intermediate arrays, leading to increased memory allocation and deallocation.
• Spread:
  • Pros: Efficient way to add multiple elements to an existing array without creating new arrays.
  • Cons: May lead to performance issues if the spread operator is used in loops or with large datasets.

Library/Functionality Used:

None of these test cases explicitly use any libraries beyond what is built-in to JavaScript. However, some modern browsers like Chrome may optimize certain methods for better performance (e.g., the concat() method).

Special JS Feature/Syntax:

The spread operator (...) is a feature introduced in ECMAScript 2015. It allows adding elements from one array to another without creating an intermediate array.

Other Alternatives:

If you wanted to test other approaches, some alternatives could include:

• Using splice() method instead of push()
• Using Array.prototype.slice() method to create a new array before appending
• Using libraries like Lodash or Ramda for more efficient array manipulation

However, these alternatives would require modifying the benchmark code and may not provide meaningful results compared to the original test cases.