Benchmark: for length vs for in

Script Preparation code:

var arr=Array(1000).fill('');var arrc=[];

Tests:

for length
for(let i=0;i<arr.length;i++){ arrc.push(arr[i]); }
for in
for(let i in arr){ arrc.push(arr[i]); }

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
for length
for in

Fastest: N/A

Slowest: N/A

Latest run results:

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

User agent: Mozilla/5.0 (iPhone; CPU iPhone OS 17_6 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.6 Mobile/15E148 Safari/604.1

Browser/OS: Mobile Safari 17 on iOS 17.6

View result in a separate tab

Test name	Executions per second
for length	8012.9 Ops/sec
for in	5717.1 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 JavaScript microbenchmark on MeasureThat.net.

**Benchmark Overview**

The benchmark compares two approaches for iterating over an array: using `for` loop with explicit indexing (`arr.length`) versus using `for...in` loop.

**Script Preparation Code**

The script preparation code is used to set up the testing environment. It creates an array `arr` of 1000 empty strings and another array `arrc` that will store the results.

```javascript
var arr = Array(1000).fill('');
var arrc = [];
```

In this case, both `arr` and `arrc` are arrays, so there's no difference in their types. However, the choice of which one to use as the iteration variable could affect performance.

**Benchmark Test Cases**

There are two test cases:

1. **"for length"`**: This test case uses a `for...let` loop with explicit indexing (`arr.length`) to iterate over the array.
```javascript
for (let i = 0; i < arr.length; i++) {
    arrc.push(arr[i]);
}
```

2. **"for in"`**: This test case uses a `for...in` loop to iterate over the array.
```javascript
for (let i in arr) {
    arrc.push(arr[i]);
}
```

**Pros and Cons of each approach:**

*   **`for length`**:
    *   Pros:
        *   More predictable and efficient, as it uses explicit indexing.
        *   Fewer array traversals required.
    *   Cons:
        *   May not be compatible with certain data structures (e.g., objects).
*   **`for in`**:
    *   Pros:
        *   Compatible with certain data structures (e.g., objects, arrays as objects).
        *   Can be useful when working with object properties.
    *   Cons:
        *   Less predictable and efficient, as it iterates over the array's prototype chain.
        *   May require additional checks to avoid iterating over inherited properties.

**Library**

None.

**Special JavaScript Features or Syntax**

There are no special features or syntax mentioned in this benchmark. It only uses standard JavaScript language features.

**Alternatives**

Other alternatives for iterating over arrays include:

*   **`Array.prototype.forEach()`**: This method is similar to `for...in`, but it's more efficient and doesn't require explicit indexing.
    ```javascript
arrc = arr.forEach(function (element) {
    return element;
});
```
*   **`Array.prototype.map()`**: This method returns a new array with the results of applying a provided function on every element in this array.
    ```javascript
arrc = arr.map(function (element) {
    return element;
});
```
*   **Manual iteration using `while` loop or `Array.prototype.forEach()`**: These methods allow for more control over the iteration process but can be less readable and efficient.

Keep in mind that the choice of iteration method depends on the specific requirements of your project. In general, `for...in` is suitable for iterating over objects or arrays as objects, while `for length` is better suited for arrays.

Related benchmarks:

for length vs for in (version: 0)

Comparing performance of: for length vs for in

Created: one year ago by: Guest

Jump to the latest result

for length

for in

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

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