Benchmark: empty an array in JavaScript - [] vs setting length

Tests:

Set length to zero
var size = 10000; var arr2 = []; for (var i = 0; i < size; i++){ arr2.push(i); } arr2.length = 0;
New empty array
var size = 10000; var arr3 = []; for (var i = 0; i < size; i++){ arr3.push(i); } arr3 = [];

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
Set length to zero
New empty array

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: 14 hours ago)

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:137.0) Gecko/20100101 Firefox/137.0

Browser/OS: Firefox 137 on Windows

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Test name	Executions per second
Set length to zero	52476.4 Ops/sec
New empty array	70403.7 Ops/sec

Autogenerated LLM Summary (model gemma2:9b, generated 6 months ago):

LLMs can make mistakes. Check important info.

This benchmark on MeasureThat.net compares two ways to empty an array in JavaScript:

**Option 1: `arr.length = 0;`**  (Test Name: "Set length to zero")

* **Description:** This method directly sets the `length` property of the array to zero, effectively removing all elements without iterating through them.

**Option 2: `arr = [];`** (Test Name: "New empty array")

* **Description:**  This approach creates a brand new, empty array and assigns it to the variable that previously held the original array. This essentially discards the old array's contents and replaces it with a fresh one.

**Pros and Cons:**

* **`arr.length = 0;` (Set length):**
    * **Pros:** Generally faster because it avoids creating a new array object, potentially saving memory.  It directly modifies the existing array structure.
    * **Cons:** Can be less intuitive if you're thinking about completely replacing the array's contents.

* **`arr = [];` (New array):**
    * **Pros:**  More explicit in its intention to create a completely new, empty array. May be easier to understand for beginners. 
    * **Cons:**  Potentially slower due to the garbage collection process involved when creating a new array object and discarding the old one.

**Other Considerations:**

* **Garbage Collection:** JavaScript's garbage collector will handle cleaning up the old array references in both cases, but the "New empty array" approach might trigger garbage collection sooner, leading to slight performance differences depending on your environment.
* **Use Case:** The best choice often depends on your specific needs. If you need to preserve the original array object (e.g., for potential future reference or modifications), setting the length is more efficient. If you truly need a fresh, completely new array, creating a new one might be clearer and less error-prone.

Let me know if you'd like to explore any of these aspects in more detail!

Related benchmarks:

empty an array in JavaScript - [] vs setting length (version: 0)

Comparing performance of: Set length to zero vs New empty array

Created: 2 years ago by: Guest

Jump to the latest result

Set length to zero

New empty array

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model gemma2:9b, generated 6 months ago):