Benchmark: JS lastIndexOf vs indexOf

Script Preparation code:

var str = '';
var i = 0;

while (i <= 1E5) str += `${i++}`;

Tests:

Array.lastIndexOf
const item = str.lastIndexOf(1E5);
Array.indexOf
const index = str.indexOf(1E5);

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
Array.lastIndexOf
Array.indexOf

Fastest: N/A

Slowest: N/A

Latest run results:

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

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.0.0 Safari/537.36

Browser/OS: Chrome 131 on Windows

View result in a separate tab

Test name	Executions per second
Array.lastIndexOf	10767309.0 Ops/sec
Array.indexOf	2088.0 Ops/sec

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

LLMs can make mistakes. Check important info.

I'd be happy to help you understand what's being tested in this JavaScript microbenchmark.

**Overview**

The benchmark compares the performance of two methods on an array: `lastIndexOf` and `indexOf`. Both methods are used to find the last index of a specific element (in this case, 100,000) within an array that starts with numbers from 0 to 100,000.

**Options Compared**

The two options being compared are:

1. **`lastIndexOf`**: This method finds the highest index at which a specified value can be found in the array.
2. **`indexOf`**: This method finds the index of the first occurrence of a specified value in the array.

**Pros and Cons**

* **`lastIndexOf`**:
	+ Pros: Can be faster for large arrays because it stops searching as soon as it finds the target element, which can reduce overhead.
	+ Cons: Returns -1 if the element is not found, which may lead to false positives or unnecessary calculations in some cases. Also, if you want to find the last occurrence of an element that appears multiple times, `lastIndexOf` might be slower due to its implementation.
* **`indexOf`**:
	+ Pros: Returns 0 as soon as it finds the target element, which can reduce overhead for finding repeated elements. Also, it's generally more concise and readable than `lastIndexOf`.
	+ Cons: Can be slower for large arrays because it searches the entire array even if it finds the target element early.

**Library and Special JS Features**

There is no specific library being used in this benchmark. However, note that both methods are part of the standard JavaScript API, which means they are supported by most modern browsers and environments without additional dependencies.

No special JavaScript features are being tested or highlighted in this benchmark.

**Other Considerations**

When choosing between `lastIndexOf` and `indexOf`, consider the specific use case:

* Use `lastIndexOf` when you need to find the last occurrence of an element, and it's not a concern if it doesn't exist.
* Use `indexOf` when you want to find the first occurrence of an element, or when finding repeated elements is more efficient.

**Alternatives**

If you're interested in comparing other methods for finding indices in arrays, some alternatives might include:

1. **Binary search**: This method can be faster than linear search but requires a sorted array.
2. **Splice-based search**: Some implementations use `splice()` to remove the element from the start of the array and then check if it's present, which can be slower for small arrays.

Keep in mind that this benchmark focuses specifically on comparing `lastIndexOf` and `indexOf`, so exploring alternative methods might not provide meaningful results without further customization.

Related benchmarks:

JS lastIndexOf vs indexOf (version: 0)

JS lastIndexOf vs indexOf

Comparing performance of: Array.lastIndexOf vs Array.indexOf

Created: 2 years ago by: Guest

Jump to the latest result

Array.lastIndexOf

Array.indexOf

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):