Benchmark: .startsWith() vs. RegExp for short strings and short prefixes

Script Preparation code:

const regexp = new RegExp(/^\-\-/);
const prefix = "--";
const str = "--some-var";
const numTests = 10000;

Tests:

.startsWith()
let i = 0; while (i < numTests) { str.startsWith(prefix); i++; }
RegExp
let i = 0; while (i < numTests) { regexp.test(str); i++; }

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
.startsWith()
RegExp

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: 6 days ago)

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.3 Safari/605.1.15

Browser/OS: Safari 18 on Mac OS X 10.15.7

View result in a separate tab

Test name	Executions per second
.startsWith()	11413.1 Ops/sec
RegExp	159610.1 Ops/sec

Autogenerated LLM Summary (model gpt-4o-mini, generated 6 days ago):

LLMs can make mistakes. Check important info.

The benchmark provided tests two different methods for determining whether a string starts with a specific prefix: using the `.startsWith()` method and leveraging a regular expression (RegExp).

### Options Compared

1. **`.startsWith()` Method**
   - **Description:** The `.startsWith()` method is a built-in JavaScript method that checks if a given string begins with a specified sequence of characters, returning a boolean value (`true` or `false`).
   - **Usage in the Benchmark:** The benchmark iterates `numTests` (10,000) times, calling `str.startsWith(prefix)` each time, where `str` is the string being tested and `prefix` is the substring to check against.
   - **Pros:**
     - **Simplicity:** The syntax is straightforward and easy to read.
     - **Performance:** For short strings and prefixes, `.startsWith()` is generally faster and more optimized in most JavaScript engines.
     - **Clarity:** Intuitive for developers as it clearly indicates the intention of checking the prefix.
   - **Cons:**
     - **Limited Flexibility:** Designed specifically for prefix checks; not suitable for patterns beyond the straightforward case.

2. **Regular Expression (RegExp)**
   - **Description:** A regular expression is a sequence of characters that form a search pattern, commonly used for string matching and manipulation. In this benchmark, a RegExp object is created to test if the string starts with the specified prefix.
   - **Usage in the Benchmark:** The benchmark calls `regexp.test(str)` in the same loop for `numTests`.
   - **Pros:**
     - **Flexibility:** Regular expressions can handle more complex patterns, allowing for sophisticated matching criteria beyond simple prefixes.
     - **Reuse of patterns:** Once a regular expression is compiled, it can be reused multiple times efficiently.
   - **Cons:**
     - **Complexity:** Regular expressions can be difficult for developers to read and understand, especially if the pattern is intricate.
     - **Performance Overhead:** For straightforward prefix checks, RegExp can be slower compared to the simple `.startsWith()` due to the added overhead of pattern matching.

### Results Comparison

The benchmark results show the following performance:

- **RegExp Executions Per Second:** 159,610.06
- **.startsWith() Executions Per Second:** 11,413.05

From the results, we can observe that regular expressions are significantly faster for this specific test case with short strings and prefixes. This can be surprising, as one might expect that the simpler method, `.startsWith()`, would be quicker due to its straightforward purpose.

### Considerations

- **Use Cases:** For most simple prefix checks, `.startsWith()` is preferred due to its clarity and intended usage. However, if there’s a need for more complex validation where patterns are involved, then RegExp becomes the necessary choice, despite its complexity and potentially lower readability.
- **Performance Implications:** JavaScript engines are continuously optimized; therefore, the performance metrics can vary across different browsers and versions. It's essential to consider these benchmarks in the context of the specific environment in which the code will run.

### Other Alternatives

- **Substring Checking:** One could also use `str.indexOf(prefix) === 0` to check if a string starts with a given prefix. This method is less readable than `.startsWith()` and may be less efficient in terms of performance.
- **Slice Method:** Another alternative would be to slice the string (`str.slice(0, prefix.length) === prefix`). While this can work, it also reduces readability and may have performance implications similar to `indexOf`.

### Conclusion

The choice between `.startsWith()` and RegExp encapsulates a balance between readability, performance, and complexity. It’s essential for developers to consider the specific use case and context when selecting the method for performance string checks. Regular expressions, while powerful, should be reserved for cases where their capabilities are needed, rather than for simple checks, where `.startsWith()` is usually sufficient and clearer.

Related benchmarks:

.startsWith() vs. RegExp for short strings and short prefixes (version: 1)

Comparing performance of: .startsWith() vs RegExp

Created: 6 days ago by: Registered User

Jump to the latest result

.startsWith()

RegExp

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model gpt-4o-mini, generated 6 days ago):