Benchmark: Array .concat() vs .unshift()

Script Preparation code:

var arr = []

Tests:

arr.unshift(42)
arr.unshift(42)
[42].concat(arr)
[42].concat(arr)

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
arr.unshift(42)
[42].concat(arr)

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: 4 years ago)

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36

Browser/OS: Chrome 83 on Mac OS X 10.15.5

View result in a separate tab

Test name	Executions per second
arr.unshift(42)	26330.9 Ops/sec
[42].concat(arr)	1664.0 Ops/sec

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

LLMs can make mistakes. Check important info.

Let's dive into the explanation of the provided benchmark.

**What is being tested?**

The benchmark measures the performance difference between two approaches to add an element to the end of an array: `arr.unshift(42)` and `[42].concat(arr)`. The goal is to determine which approach is faster.

**Options compared**

There are only two options being compared:

1. `arr.unshift(42)`: This method adds a new element to the beginning of the array. It shifts all existing elements down by one position, making room for the new element.
2. `[42].concat(arr)`: This method creates a new array with the original array as its first element and the new number `42` as its second element.

**Pros and Cons**

**1. arr.unshift(42)**

Pros:

* Efficient use of memory, as it only adds one new element to the existing array.
* Fast execution time, as it's a simple operation that only requires updating the array's length.

Cons:

* Can be slower than concatenation for very large arrays, since shifting elements can lead to slower cache performance.
* May not be suitable for arrays with many null or undefined values, as it can lead to unnecessary shifts.

**2. [42].concat(arr)**

Pros:

* Suitable for large arrays, as it creates a new array and doesn't modify the original one.
* Fast execution time, since it's just an array concatenation operation.

Cons:

* Less memory-efficient than `unshift`, as it creates a new array with two elements: the original array and the new number.
* Can be slower for small arrays due to the overhead of creating a new array.

**Library and its purpose**

In this benchmark, the JavaScript Array class is used. The Array class provides methods like `push()`, `unshift()`, and `concat()` that allow developers to manipulate array elements.

**Special JS feature or syntax**

There is no special JavaScript feature or syntax being tested in this benchmark. However, it's worth noting that this benchmark might not be representative of real-world scenarios, as it only tests two specific methods on a small array. In practice, you may encounter different performance characteristics when working with larger arrays or more complex data structures.

**Other alternatives**

If you wanted to test other approaches for adding elements to an array, some additional options could include:

* Using `Array.prototype.push()` instead of `unshift()`
* Using `Array.prototype.splice()` to insert a new element at a specific position
* Using `Array.prototype.set()` if you're working with older browsers that support this method

However, these alternatives are not being tested in this benchmark.

Related benchmarks:

Array .concat() vs .unshift() (version: 0)

Comparing performance of: arr.unshift(42) vs [42].concat(arr)

Created: 4 years ago by: Guest

Jump to the latest result

arr.unshift(42)

[42].concat(arr)

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

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