Benchmark: Object.assign vs for-in assign

Script Preparation code:

var input = {};
for (let i = 0; i < 100_000; ++i)
{
  input[`prop${i}`] = i;
}

var output = {};

Tests:

Object.assign
Object.assign(output, input);
for-in assign
for (let prop in input) { output[prop] = input[prop]; }

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
Object.assign
for-in assign

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: one year ago)

User agent: Mozilla/5.0 (X11; Linux x86_64; rv:125.0) Gecko/20100101 Firefox/125.0

Browser/OS: Firefox 125 on Linux

View result in a separate tab

Test name	Executions per second
Object.assign	293.6 Ops/sec
for-in assign	154.4 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 and explain what's being tested.

**Benchmark Overview**

The benchmark compares the performance of two approaches for assigning properties to an object: `Object.assign()` and a `for-in` loop. The test creates a large input object with 100,000 properties, each containing a unique value from 0 to 99,999. The output object is also created empty.

**Options Compared**

The benchmark tests the performance of two options:

1. **Object.assign()**: This method takes two arguments: the source object and the target object. It assigns all enumerable own properties from the source object to the target object.
2. **for-in loop**: This approach uses a `for...in` loop to iterate over the input object's properties, assigning each property value to the corresponding key in the output object.

**Pros and Cons of Each Approach**

1. **Object.assign()**:
	* Pros: Concise, readable, and widely supported.
	* Cons: Can be slower due to the overhead of method calls and potential optimizations in other JavaScript implementations.
2. **for-in loop**:
	* Pros: Low-level control, can be optimized for performance, and is often preferred by developers familiar with the `for...in` syntax.
	* Cons: Less readable and more prone to errors, especially when dealing with large numbers of properties.

**Library Used**

None, as this benchmark only uses built-in JavaScript functions.

**Special JS Feature/Syntax (Not Applicable)**

No special features or syntax are used in this benchmark.

**Alternative Approaches**

Other ways to assign properties to an object include:

1. **Array.prototype.map()**: Assigns values to keys using the `map()` method and an arrow function.
2. **Array.prototype.forEach() with Object.keys()**: Uses `forEach()` and `Object.keys()` to iterate over the input object's properties and assign them to the output object.
3. **Destructuring assignment**: Uses destructuring syntax to assign properties from the input object to the output object.

**Benchmark Preparation Code Explanation**

The script preparation code creates an empty input object (`input`) and populates it with 100,000 properties using a `for...in` loop. The code then defines an empty output object (`output`). This setup allows both benchmarked methods to be tested in isolation.

**Individual Test Cases**

Each test case contains a single benchmark definition:

1. **Object.assign()**: Assigns the input object to the output object using `Object.assign()`.
2. **for-in loop**: Iterates over the input object's properties and assigns each value to the corresponding key in the output object using a `for...in` loop.

**Latest Benchmark Result**

The results show the number of executions per second for each browser (Firefox 125) on a desktop, Linux device:

* **Object.assign()**: 293.5997619628906 executions/second
* **for-in assign**: 154.36810302734375 executions/second

This result suggests that `Object.assign()` is significantly faster than the `for-in` loop approach in this specific benchmark.

Related benchmarks:

Object.assign vs for-in assign (version: 0)

Comparing performance of: Object.assign vs for-in assign

Created: one year ago by: Registered User

Jump to the latest result

Object.assign

for-in assign

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

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