Benchmark: Big Array prepend

Script Preparation code:

var a;
var prependArray = function(value, oldArray) {
    var i, len = oldArray.length + 1,
        newArray = new Array(len);
    newArray[0] = value;
    for (i = 0; i < len; ++i) {
        newArray.push(oldArray[i]);
    }
    return newArray;
}

Tests:

Spread Operator
a = new Array(10000).fill().map((_, i) => i) a = [0, ...a];
Unshift
a = new Array(10000).fill().map((_, i) => i) a.unshift(0);
Unshift without mutation
a = new Array(10000).fill().map((_, i) => i) a.slice(0).unshift(0);
Prepend function
a = new Array(10000).fill().map((_, i) => i) prependArray(0, a);

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
Spread Operator
Unshift
Unshift without mutation
Prepend function

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: one month ago)

User agent: Mozilla/5.0 (iPhone; CPU iPhone OS 18_3_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.3.1 Mobile/15E148 Safari/604.1

Browser/OS: Mobile Safari 18 on iOS 18.3.2

View result in a separate tab

Test name	Executions per second
Spread Operator	28927.7 Ops/sec
Unshift	34056.5 Ops/sec
Unshift without mutation	27897.1 Ops/sec
Prepend function	11977.0 Ops/sec

Autogenerated LLM Summary (model llama3.2:3b, generated 6 months ago):

LLMs can make mistakes. Check important info.

I'll break down the provided benchmark and explain what's being tested, along with the pros and cons of different approaches.

**Benchmark Definition**

The benchmark definition represents a JavaScript microbenchmark. It defines a small piece of code that will be executed repeatedly to measure its performance. In this case, there are four test cases:

1. `Spread Operator`: The code creates an array of 10,000 elements using `Array(10000).fill().map((_, i) => i)` and then uses the spread operator (`[...a]`) to create a new array with the first element set to 0.
2. `Unshift`: The code creates an array of 10,000 elements using `Array(10000).fill().map((_, i) => i)` and then uses the `unshift` method to add 0 as the first element.
3. `Unshift without mutation`: This test case is similar to the previous one, but it creates a new array instead of modifying the original one using `slice(0)`.
4. `Prepend function`: The code defines a custom function `prependArray` that takes two arguments: `value` and `oldArray`. It returns a new array with the `value` as the first element and the rest of the elements from `oldArray`.

**Test Cases**

Each test case represents a different approach to adding an element to the beginning of an array. Here's what's being tested:

* **Spread Operator**: The spread operator is used to create a new array by taking the elements from the original array and adding a new element at the beginning.
	+ Pros: Fast, efficient way to add elements to the beginning of an array. It creates a new array, which can be beneficial for performance-critical code.
	+ Cons: Creates a new array, which can lead to memory allocation overhead.
* **Unshift**: The `unshift` method modifies the original array by adding a new element at the beginning.
	+ Pros: Modifies the original array, which can be beneficial for reducing memory allocation overhead.
	+ Cons: Slower than spread operator due to the need to modify the existing array.
* **Unshift without mutation**: This test case creates a new array instead of modifying the original one using `slice(0)`.
	+ Pros: Similar performance benefits as unshift, but avoids modifying the original array.
	+ Cons: Creates an additional copy of the original array.
* **Prepend function**: The custom `prependArray` function adds an element to the beginning of the array using a loop.
	+ Pros: Customizable implementation that can be optimized for specific use cases.
	+ Cons: Generally slower than spread operator and unshift due to the need for iteration.

**Library**

There is no explicit library mentioned in the benchmark definition. However, some test cases may rely on built-in JavaScript methods or functions, such as `Array.prototype.unshift` or `Array.prototype.slice`.

**Special JS Feature/Syntax**

None of the test cases use any special JavaScript features or syntax beyond what's described above.

**Alternatives**

Other approaches to adding an element to the beginning of an array could include:

* Using `concat` instead of spread operator: `a.concat([0])`
* Using `push` and then shifting the elements using `shift`: `a.push(0); a.shift()`
* Using a custom implementation with a different data structure, such as a linked list.

Keep in mind that these alternatives may have varying performance characteristics depending on the specific use case.

Related benchmarks:

Big Array prepend (version: 0)

Comparing performance of: Spread Operator vs Unshift vs Unshift without mutation vs Prepend function

Created: 4 years ago by: Guest

Jump to the latest result

Spread Operator

Unshift

Unshift without mutation

Prepend function

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated 6 months ago):