Benchmark: length assignment during loop

Script Preparation code:

var arr = [];

for(let i=0; i<=1000; ++i){
  arr.push(i);
}

Tests:

assignment
for (let i = 0, length = arr.length; i < length; i++){ }
no assignment
for (let i = 0; i < arr.length; i++){ }

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
assignment
no assignment

Fastest: N/A

Slowest: N/A

Latest run results:

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

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/122.0.0.0 Safari/537.36

Browser/OS: Chrome 122 on Mac OS X 10.15.7

View result in a separate tab

Test name	Executions per second
assignment	2460290.5 Ops/sec
no assignment	32269.2 Ops/sec

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

LLMs can make mistakes. Check important info.

Let's break down the provided JSON benchmark definition and test cases.

**Benchmark Definition:**

The benchmark is designed to measure the performance difference between two approaches in JavaScript:

1. `length assignment during loop`: This approach involves assigning the length of the array to a variable inside the loop.
2. `no assignment`: This approach does not assign the length of the array to a variable inside the loop.

**Script Preparation Code:**

The script preparation code is provided as:
```javascript
var arr = [];
for (let i = 0; i <= 1000; ++i) {
  arr.push(i);
}
```
This code creates an empty array `arr` and then pushes 1000 elements onto it using a loop.

**Html Preparation Code:**

There is no HTML preparation code provided, so we'll assume that the benchmark runs in a headless browser environment.

**Options Compared:**

The two options being compared are:

1. **Length assignment**: Assigning the length of the array to a variable inside the loop.
2. **No assignment**: Not assigning the length of the array to a variable inside the loop.

**Pros and Cons of Each Approach:**

**Length Assignment:**

Pros:

* Can be faster because it avoids a potential additional memory allocation or copying operation required when pushing elements onto an array.
* May provide better cache locality, as the loop counter is accessible directly from the array length property.

Cons:

* Requires accessing the `length` property of the array, which may incur a small overhead due to the JavaScript engine's need to resolve the variable.
* Can lead to performance regressions if the browser or engine misoptimizes this specific use case.

**No Assignment:**

Pros:

* Does not require accessing the `length` property, potentially avoiding any additional overhead or overhead resolution steps.
* May allow for better compiler optimizations and caching, as the loop counter is not explicitly stored in a variable.

Cons:

* May lead to slower performance due to the need to repeatedly access the array length property within the loop.
* Can result in less cache locality, as the loop counter is not directly accessible from the array length property.

**Library Used:**

The benchmark uses the `arr` array object, which is a built-in JavaScript object. The `push()` method is used to add elements to the end of the array.

No external libraries are required for this benchmark.

**Special JS Feature/Syntax:**

There are no special JavaScript features or syntax being tested in this benchmark.

**Alternatives:**

Other alternatives that could be used to measure the performance difference between these two approaches might include:

* Using a different data structure, such as an array buffer or an array-like object.
* Using a loop unrolling technique to increase the number of iterations per loop iteration.
* Measuring the performance of the `push()` method with and without caching the result.

It's worth noting that MeasureThat.net is designed to provide a simple and repeatable way to measure JavaScript performance, and it has already implemented these different approaches for comparison.

Related benchmarks:

length assignment during loop (version: 0)

Comparing performance of: assignment vs no assignment

Created: 2 years ago by: Registered User

Jump to the latest result

assignment

no assignment

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

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