Measuring the performance of different ways to allocate and manipulate arrays in JavaScript is a common task.

Benchmark Overview

The benchmark, named "TestArrayAllocationv2", consists of 7 test cases that vary in how they allocate and populate an array with objects. The goal is to determine which approach yields the best performance.

Options Compared

Here are the different approaches compared:

1. Direct indexing: array[i] = getObj(); (test cases: 1, 2, 4, 5)
   • This method uses direct indexing to access and assign values to elements in the array.
2. Loops with explicit variable assignment: let i; for (i in array) array[i] = getObj(); (test case: 6)
   • This method uses a loop with an explicit variable declaration to iterate over the array and assign values to its elements.
3. For-in loops: for (const i in array) array[i] = getObj(); (test case: 4, 5 is similar but without let i; )
   • This method uses a for-in loop to iterate over the array's properties and assign values to them.
4. Array methods: array.map(_ => getObj()); (test case: 6)
   • This method uses the map() function to create a new array with the same number of elements, where each element is an object created by calling getObj().
5. Push assignment: arr3.push(getObj()); (test case: 7)
   • This method adds objects to the end of an existing array using the push() function.

Pros and Cons

Here's a brief summary of each approach:

1. Direct indexing: Fast and simple, but may lead to better cache locality due to sequential access.
2. Loops with explicit variable assignment: May be slower than direct indexing, but still relatively fast. The use of let i allows the loop variable to "fall through" to the next iteration without declaring it again.
3. For-in loops: Can lead to better cache locality due to the random access pattern, which can be beneficial for modern CPUs with high cache hierarchies. However, may be slower than direct indexing in some cases.
4. Array methods: Generally slower than direct indexing or for-in loops, as it creates a new array and assigns elements to it.
5. Push assignment: May lead to slower performance due to the overhead of adding an element to the end of an existing array.

Library Considerations

None of these test cases rely on any external libraries that would affect the benchmark results.

Special JS Feature or Syntax

There are a few things worth noting:

• The use of _ as the loop variable in array.map(_ => getObj()); is a convention to indicate that the variable name is not important.
• The let i; syntax in some test cases allows for the loop variable to "fall through" to the next iteration without declaring it again.

Other Alternatives

Some alternative approaches that could be included in this benchmark are:

• Using Array.prototype.forEach() instead of a traditional loop
• Using slice() or other array methods to create a new array and assign elements to it
• Using WebAssembly (WASM) or other low-level, compiled languages to write performance-critical loops