Benchmark: `Array.slice(0, N)` vs `Array.length = N`

Script Preparation code:

var arr = Array.from(Array(100).keys());

Tests:

`Array.slice(0, N)`
arr.slice(0, 20);
`Array.length = N`
arr.length = 20;
`Object.assign([], arr)` with `Array.length = N`
var newArr = Object.assign([], arr); newArr.length = 20;

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
`Array.slice(0, N)`
`Array.length = N`
`Object.assign([], arr)` with `Array.length = N`

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated one month ago):

LLMs can make mistakes. Check important info.

Let's break down the benchmark and explain what's being tested.

**Benchmark Definition**

The benchmark is defined in JSON format, which represents a comparison between three different approaches to set the length of an array:

1. `Array.slice(0, N)` - This approach creates a new array by slicing the original array from index 0 to the specified length (N).
2. `Array.length = N` - This approach directly sets the length property of the original array.
3. `Object.assign([], arr) with Array.length = N` - This approach uses the `Object.assign()` method to create a new object from an empty array and then assigns the desired length to the resulting array.

**Options Compared**

The benchmark compares these three approaches in terms of their performance, specifically:

* Execution speed
* Number of executions per second

**Pros and Cons of Each Approach**

1. **`Array.slice(0, N)`**:
	* Pros: Creates a new array with the desired length, which can be useful for certain use cases.
	* Cons: May incur additional overhead due to creating a new array object, which can lead to slower performance compared to direct property assignment.
2. **`Array.length = N`**:
	* Pros: Directly sets the length property of the original array, which is likely to be faster since it doesn't involve creating a new object or array.
	* Cons: Can lead to issues if the length is set incorrectly, as it will affect all elements in the array.
3. **`Object.assign([], arr) with Array.length = N`**:
	* Pros: Uses a more functional programming approach, which can be beneficial for some use cases.
	* Cons: Creates an unnecessary intermediate object and assigns its length to the resulting array, adding overhead.

**Library Usage**

None of the benchmarked approaches rely on external libraries. The `Array` object is used directly in each case.

**Special JS Feature/Syntax**

None of the benchmarked approaches utilize special JavaScript features or syntax beyond standard language constructs.

**Other Alternatives**

If you need to set an array length, alternative approaches could include:

* Using a library like Lodash, which provides a `_.setLength()` function.
* Utilizing a specialized data structure, such as a sparse array, which can be created using `Array.prototype.set()` or similar methods.
* Implementing a custom length setter using a closure or a proxy object.

Keep in mind that the choice of approach depends on the specific use case and performance requirements. The benchmark results provided by MeasureThat.net can help inform these decisions.

Latest run results:

Run details: (Test run date: 29 days ago)

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/129.0.0.0 Safari/537.36

Browser/OS: Chrome 129 on Windows

View result in a separate tab

Test name	Executions per second
`Array.slice(0, N)`	15604940.0 Ops/sec
`Array.length = N`	13238485.0 Ops/sec
`Object.assign([], arr)` with `Array.length = N`	406114.7 Ops/sec

Related benchmarks:

`Array.slice(0, N)` vs `Array.length = N` (version: 0)

Comparing performance of: `Array.slice(0, N)` vs `Array.length = N` vs `Object.assign([], arr)` with `Array.length = N`

Created: 4 years ago by: Guest

Jump to the latest result

`Array.slice(0, N)`

`Array.length = N`

`Object.assign([], arr)` with `Array.length = N`

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated one month ago):