Benchmark: Native slice vs copy

Script Preparation code:

function copy (a) {
  const l = a.length
  const b = new Array(l)
  for (let i = 0; i < l; ++i) {
    b[i] = a[i]
  }
  return b
}

function copyNative (a) {
  return a.slice(0)
}

Tests:

Native
var arr = [1,2,3,4,5,6] var res = copyNative(arr)
For Loop
var arr = [1,2,3,4,5,6] var res = copy(arr)

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
Native
For Loop

Fastest: N/A

Slowest: N/A

Latest run results:

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Autogenerated LLM Summary (model gemma2:9b, generated one year ago):

LLMs can make mistakes. Check important info.

This benchmark tests the performance of two methods for copying an array in JavaScript:

**Options being compared:**

* **`array.slice(0)`:** This uses the native `slice()` method to create a shallow copy of the array from index 0 to the end.  It's concise and generally considered efficient.
* **For loop with preallocation:** This involves manually iterating through the original array using a `for` loop and creating a new array (`b`) with the same length beforehand. Each element from the original array is then copied into the new array.

**Pros and Cons:**

* **`array.slice(0)` (Native):**
    * **Pros:**  Very concise, often optimized by JavaScript engines for performance.
    * **Cons:** Can be slightly less performant in some specific edge cases compared to a carefully implemented for loop.
* **For Loop with Preallocation:**
    * **Pros:** Potentially faster in very specific cases where the engine's internal optimizations for `slice()` aren't as efficient (though this is uncommon).  Offers more fine-grained control over the copying process if needed.
    * **Cons:** More verbose code, can be less readable than `slice()`.

**Other Considerations:**

* **Array Size:** For small arrays, the performance difference between the two methods will likely be negligible. As array size increases, the potential performance advantage of  `slice()` might become more apparent.
* **Engine Optimizations:** JavaScript engine implementations can vary significantly. Some engines may have highly optimized `slice()` implementations that outperform a carefully written for loop in most scenarios.

**Alternatives:**

While not directly compared in this benchmark, there are other array manipulation techniques:

* **`Array#concat()`:**  Creates a new array by concatenating existing arrays. It's generally slower than `slice()` for creating simple copies.
* **Libraries (e.g., Lodash):** Libraries often provide optimized functions like `_.cloneDeep()` for deep copying objects and arrays, which might be more suitable for complex data structures.

Let me know if you have any other questions!

Related benchmarks:

Native slice vs copy (version: 0)

Array#slice vs for loop with array preallocation

Comparing performance of: Native vs For Loop

Created: 8 years ago by: Guest

Jump to the latest result

Native

For Loop

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

No previous run results

Autogenerated LLM Summary (model gemma2:9b, generated one year ago):