Benchmark: lastIndexOf vs negativeSlice

Script Preparation code:

var lookFor = "planet";
var text = "Hello planet earth, you are a great planet";

Tests:

length is match plus length
(text.length === (text.lastIndexOf(lookFor)+lookFor.length))
slice
(text.slice(-lookFor.length) === lookFor)

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
length is match plus length
slice

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated 26 days ago):

LLMs can make mistakes. Check important info.

Let's break down the benchmark and its test cases to understand what's being tested.

**Benchmark Overview**

The benchmark compares two approaches for finding a substring in a string:

1. Using `lastIndexOf` with the original length of the substring (`lastIndexOf + lookFor.length`)
2. Using `slice(-lookFor.length)` (which returns a new string containing all characters from the end of the original string to the start of the look-for substring)

**Script Preparation Code**

The script preparation code sets two variables:

* `lookFor`: a string literal with the value `"planet"`
* `text`: a string variable initialized with the value `"Hello planet earth, you are a great planet"`

This code creates a context where both test cases can run their respective benchmarks.

**Test Cases**

There are two individual test cases in the benchmark:

1. **"length is match plus length"`**
	* The benchmark definition is: `text.length === (text.lastIndexOf(lookFor) + lookFor.length)`
	* This test case checks if the length of the original string (`text`) matches the sum of the last index of the look-for substring and its own length.
2. **"slice"`**
	* The benchmark definition is: `text.slice(-lookFor.length) === lookFor`
	* This test case checks if using the `slice` method with a negative offset (equivalent to `-lookFor.length`) returns the same result as simply comparing `lookFor` with the original string.

**Library/Features**

In this benchmark, we don't see any explicit use of external libraries. However, it's worth noting that some JavaScript features like `lastIndexOf`, `slice`, and template literals (`\r\n`) are part of the standard library.

**Special JS Features/Syntax**

This benchmark uses a feature called "template literals" (introduced in ECMAScript 2015). Template literals allow you to embed expressions inside string literals using backticks (`) instead of the traditional double quotes. The `\r\n` escape sequence is also specific to some JavaScript engines and can be considered a non-standard syntax.

**Alternatives**

If we were to rewrite this benchmark, we could consider alternative approaches:

1. **Using `indexOf()`**: Instead of `lastIndexOf`, we could use `indexOf()` to find the last index of the look-for substring.
2. **Using regex**: We could also use regular expressions (`/regex/`) to find the last occurrence of the look-for substring.

However, these alternatives might not be as straightforward or efficient as the original code using `lastIndexOf` and `slice`.

**Pros and Cons**

Here's a brief pros and cons analysis for each approach:

* **Using `lastIndexOf` with original length**: Pros - simple and efficient. Cons - might be slower than other approaches due to potential overflows.
* **Using `slice(-lookFor.length)`: Pros - efficient and concise. Cons - assumes the string is long enough, which might not always be the case.
* **Using `indexOf()`**: Pros - similar performance to `lastIndexOf`. Cons - might not work correctly for negative indices or edge cases.
* **Using regex**: Pros - flexible and powerful. Cons - can be slower due to regular expression overhead.

Keep in mind that these are just general pros and cons, and the actual performance differences between these approaches may vary depending on specific use cases and JavaScript engines.

Latest run results:

Run details: (Test run date: 2 years ago)

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.0.0 Safari/537.36

Browser/OS: Chrome 103 on Windows

View result in a separate tab

Test name	Executions per second
length is match plus length	2284867.8 Ops/sec
slice	3211242.0 Ops/sec

Related benchmarks:

lastIndexOf vs negativeSlice (version: 0)

Comparing performance of: length is match plus length vs slice

Created: 2 years ago by: Guest

Jump to the latest result

length is match plus length

slice

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated 26 days ago):