Benchmark: add many elements to beginning of array: unshift vs splice

Script Preparation code:

var data = [];
for (let i = 0; i < 10000; ++i) data[i] = i;
var add = [];
for (let i = 0; i < 100; ++i) add[i] = -i;

Tests:

unshift
data.unshift(...add);
splice
data.splice(0, 0, ...add)

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
unshift
splice

Fastest: N/A

Slowest: N/A

Latest run results:

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

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/118.0

Browser/OS: Firefox 118 on Windows

View result in a separate tab

Test name	Executions per second
unshift	5409.1 Ops/sec
splice	109.7 Ops/sec

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

LLMs can make mistakes. Check important info.

I'd be happy to help explain the provided JavaScript microbenchmark.

**Benchmark Definition**

The benchmark is designed to compare two approaches for adding multiple elements to the beginning of an array in JavaScript: `unshift` and `splice`. The benchmark creates an array `data` with 10,000 elements, each containing a unique integer value from 0 to 9,999. Then, it creates another array `add` with 100 elements, each containing the negation of its index.

**Script Preparation Code**

The script preparation code is provided as part of the benchmark definition:
```javascript
var data = [];
for (let i = 0; i < 10000; ++i) data[i] = i;
var add = [];
for (let i = 0; i < 100; ++i) add[i] = -i;
```
This code initializes the `data` and `add` arrays with the specified values.

**Options Compared**

The benchmark compares two options:

1. **`unshift`**: The `unshift` method adds one or more elements to the beginning of an array and returns the new length of the array.
2. **`splice`**: The `splice` method removes elements from an array and adds new elements in their place, shifting all subsequent elements down by the number of elements removed.

**Pros and Cons**

*   **`unshift`**:
    *   Pros: More efficient for adding elements to the beginning of an array.
    *   Cons: May be slower than `splice` when dealing with large arrays or frequent updates.
*   **`splice`**:
    *   Pros: Can be used to remove elements from the array while still adding new ones, making it a more versatile option.
    *   Cons: Slower and less efficient for simply adding elements to the beginning of an array.

**Library Usage**

There is no explicit library usage in this benchmark. However, the `Array.prototype.unshift` and `Array.prototype.splice` methods are built-in JavaScript methods that operate on arrays.

**Special JS Features or Syntax**

The benchmark does not use any special JavaScript features or syntax, such as async/await, Promises, or modern array methods like `map()` or `reduce()`.

**Other Alternatives**

If you were to rewrite the benchmark with alternative approaches, some options could include:

*   Using a library like Lodash, which provides optimized versions of the `unshift` and `splice` methods.
*   Implementing your own custom array insertion function using bitwise operations or other techniques to optimize performance.
*   Using parallel processing or concurrent execution to measure performance differences between `unshift` and `splice`.
*   Utilizing a just-in-time (JIT) compiler like V8 or SpiderMonkey to optimize the benchmark for specific browsers or architectures.

Keep in mind that each alternative approach would require significant changes to the benchmark code and potentially introduce new variables or assumptions.

Related benchmarks:

add many elements to beginning of array: unshift vs splice (version: 0)

Comparing performance of: unshift vs splice

Created: one year ago by: Guest

Jump to the latest result

unshift

splice

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):