Let's dive into the world of JavaScript microbenchmarks and explore what's being tested in this benchmark.

Benchmark Definition

The benchmark is designed to compare four different approaches for concatenating arrays or modifying existing arrays:

1. Array.prototype.concat(): Using the traditional concat() method.
2. Spread operator ([...]): Utilizing the new ES6 spread operator to create a new array.
3. "Copy on push" (assuming push() modifies an existing array by copying its elements and then pushing the new ones): This approach uses forEach() to iterate over the new array and append it to the existing one using concat().
4. Directly pushing (newi.forEach(i => existing.push(i))): Modifying the original array directly by iterating over the new array and pushing each element into the existing array.

Pros and Cons of Each Approach

1. Array.prototype.concat(): This approach is straightforward, but it creates a new array on every iteration, which can lead to performance issues with large datasets.
2. **Spread operator ([...] newi). This approach creates a new array without modifying the existing one, making it more memory-efficient.
3. Copy on push (copying elements using concat() and then pushing new ones): While this approach modifies an existing array, it still has the overhead of creating intermediate arrays, which might be less efficient than other methods.
4. Directly pushing (newi.forEach(i => existing.push(i))): This approach directly modifies the original array by iterating over the new array and appending each element to the existing one. It's likely the most memory-efficient option.

Library Usage

The benchmark uses Lodash, a popular JavaScript utility library, in its flat() test case:

var other = _.flatten([existing, newi]);

_flatten() is used to flatten an array of arrays into a single array. This approach is useful when you need to perform operations on nested arrays, but it may not be the most efficient way to concatenate or modify arrays.

Special JS Features/Syntax

The benchmark does not use any special JavaScript features or syntax beyond what's standard in modern browsers (ECMAScript 2015+).

Alternative Approaches

Here are some alternative approaches that could be used for this benchmark:

1. Using Array.prototype.pushAll(): This method is available in ECMAScript 2022, which allows pushing multiple values into an array at once.

existing.push(...newi);

2. Using a custom implementation with Set or Map: You could create a custom function that iterates over the new array and adds each element to a Set or Map, which would then be converted back to an array. This approach can be more efficient than using push() or concat().
3. Using a parallel processing library (e.g., Web Workers): If you have a multi-core processor, you could use Web Workers to run each test case in parallel, reducing overall execution time.

Keep in mind that these alternative approaches might not be as straightforward or well-documented as the original methods used in the benchmark.