Benchmark: groupBy spread vs mutating

Script Preparation code:

let entries = []
let keys = ['A', 'B', 'C', 'D', 'E'];

for (var i = 0; i < 1000; i++) {
  entries.push({ key: keys[i % 5], property: 'something'});
}

function getEntries() {
  return entries;
}

Tests:

Using spread operators
getEntries().reduce((groups, entry) => { const group = groups[entry.key] || []; return { ...groups, [entry.key]: [...group, entry] }; }, {});
Mutating
getEntries().reduce(function(groups, entry) { (groups[entry.key] = groups[entry.key] || []).push(entry); return groups; }, {});

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
Using spread operators
Mutating

Fastest: N/A

Slowest: N/A

Latest run results:

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

LLMs can make mistakes. Check important info.

Let's break down the provided benchmark and explain what's being tested.

**Benchmark Definition**

The benchmark is defined by two different approaches to group an array of objects based on a common key. The approach differs in how it handles the initial empty groups:

1. **Using spread operators**: This approach uses the spread operator (`...`) to create a new copy of the existing group and then concatenates the new entry to it.
2. **Mutating**: This approach modifies the existing group by adding the new entry directly.

**Options being compared**

The two options are being compared in terms of their performance:

* **Using spread operators**: This approach creates a new copy of the existing group, which can be slower due to the overhead of creating a new array and copying the elements.
* **Mutating**: This approach modifies the existing group directly, which can be faster since it avoids the overhead of creating a new array.

**Pros and Cons**

* **Using spread operators**:
	+ Pros: Creates a new copy of the existing group, which can be more predictable and easier to understand for some developers.
	+ Cons: Can be slower due to the overhead of creating a new array and copying elements.
* **Mutating**:
	+ Pros: Faster since it avoids the overhead of creating a new array.
	+ Cons: Can be less predictable and more error-prone if not handled correctly, as modifying an object's properties can have unintended consequences.

**Library**

There is no explicit library mentioned in the benchmark definition. However, the use of JavaScript's `reduce` method suggests that it's using the built-in Array.prototype methods.

**Special JS feature or syntax**

The benchmark uses JavaScript's spread operator (`...`) to create a new copy of an object. This feature was introduced in ECMAScript 2018 (ES2020) and is widely supported by modern browsers.

**Other alternatives**

If you wanted to implement this benchmark using other approaches, here are some alternative methods:

* **Using `Array.prototype.reduce()`**: Instead of using the spread operator, you can use JavaScript's built-in `reduce()` method to accumulate the groups.
* **Using a custom loop**: You can use a traditional `for` loop or `forEach` loop to iterate over the entries and accumulate the groups.
* **Using a library like Lodash**: If you're looking for a more concise solution, you can use a library like Lodash's `groupBy()` function.

For example, here's an implementation using `Array.prototype.reduce()`:
```javascript
function getEntries() {
  return entries.reduce((groups, entry) => {
    const group = groups[entry.key] || [];
    return { ...groups, [entry.key]: [...group, entry] };
  }, {});
}
```
And here's an implementation using a custom loop:
```javascript
function getEntries() {
  const groups = {};
  for (const entry of entries) {
    const key = entry.key;
    if (!groups[key]) {
      groups[key] = [];
    }
    groups[key].push(entry);
  }
  return groups;
}
```
Note that these alternative implementations may have different performance characteristics than the original benchmark.

Related benchmarks:

groupBy spread vs mutating (version: 0)

Comparing performance of: Using spread operators vs Mutating

Created: 7 years ago by: Guest

Jump to the latest result

Using spread operators

Mutating

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

No previous run results

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