Let's dive into the benchmark and explore what's being tested, compared, and the pros and cons of each approach.

What is being tested?

The benchmark is comparing three different approaches to remove elements from an array in JavaScript:

1. array.splice(0, a.length, ...s.values()): This method uses splice with a starting index of 0, removing all elements from the array and then replaces them with values from the Set object.
2. a.length = 0; for (var x of s) a.push(x);: This approach creates a new array by iterating over the values in the Set object and adding each one to the original array, effectively replacing the existing elements.
3. a.splice.apply(a, [0, a.length].concat(Array.from(s))): Similar to the first method, but without using spread syntax (...s.values()).

Options compared

The benchmark is comparing the performance of these three approaches:

• Pros and Cons:
  • array.splice(0, a.length, ...s.values()):
    • Pros: Efficient way to remove all elements from an array.
    • Cons: Can be slow for large arrays if using ... syntax (as it creates a new array with the spread operator).
  • a.length = 0; for (var x of s) a.push(x);:
    • Pros: Can be faster than splice when removing all elements, as it avoids creating an intermediate array.
    • Cons: Requires iterating over the values in the set and adding them to the original array, which can be slower for large sets.
  • a.splice.apply(a, [0, a.length].concat(Array.from(s))):
    • Pros: Similar to the first method but avoids using ... syntax, potentially making it faster for large arrays.
    • Cons: Requires creating an intermediate array, which can be slower than the splice approach.

Library and purpose

In the benchmark preparation code, a Set object is created and populated with values. A Set is a data structure in JavaScript that stores unique values, making it efficient for fast lookups and iteration. The library used here is the built-in JavaScript Set API.

Special JS feature or syntax

The benchmark uses the spread operator (...) to create an array from the Set object's values. This is a relatively modern feature introduced in ECMAScript 2018 (ES10). It allows for concise and efficient way to transform sets into arrays.

Other alternatives

If you were to implement this benchmark manually, without relying on pre-existing libraries or built-in functions, you could consider the following approaches:

• Using for loops with array indexing: a.length = 0; for (var i = 0; i < a.length; i++) { a[i] = undefined; }
• Using filter() and map(): a.splice(0, a.length); a = a.filter(x => s.has(x)).concat(s)