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

Overview

The benchmark compares two approaches for merging two object arrays: Array.prototype.concat and the new ES6 spread operator (...). The test aims to determine which approach is faster.

Options compared

There are three test cases:

1. Array.prototype.concat: This method takes an array as an argument and returns a new array that contains all elements from both arrays.
2. Spread operator (spread at head): This syntax uses the spread operator (...) to create a new array by copying elements from one or more source arrays. In this test case, the spread operator is used with response as the first argument and state as the second argument.
3. Spread operator x2 (head and tail): This variant of the spread operator uses it twice, once to combine the two input arrays (prevState and newItems) at the head and once to merge the resulting array with another array (state).

Pros and cons

1. Array.prototype.concat:
   • Pros: Widely supported, easy to understand, and well-documented.
   • Cons: Can be slower due to the overhead of creating a new array.
2. Spread operator (spread at head):
   • Pros: Modern, efficient, and often preferred by developers due to its simplicity and expressiveness.
   • Cons: May not be supported in older browsers or environments that don't have ES6 support.
3. Spread operator x2 (head and tail):
   • Pros: Can take advantage of the fact that arrays are mutable, allowing for more efficient merging operations.
   • Cons: Less intuitive than other approaches, and its usage may require careful consideration to avoid unexpected behavior.

Library and purpose

None mentioned in the benchmark definition. However, both Array.prototype.concat and the spread operator use native JavaScript functionality.

Special JS feature or syntax

The spread operator (...) is a modern JavaScript feature introduced in ES6 (ECMAScript 2015). It allows you to create new arrays by copying elements from existing arrays.

Other alternatives

Before the spread operator, developers often used Array.prototype.push and Array.prototype.concat, which also merge arrays. However, these approaches can be less efficient due to the overhead of modifying original arrays.

The benchmark results show that the spread operator (spread at head) is currently the fastest approach on this platform, followed by the spread operator x2 (head and tail) variant, and then Array.prototype.concat.