Benchmark: var only vs let + const

Script Preparation code:

var array = Array.from(Array(10000).keys());

Tests:

var only
for(var i = 1; i < array.length; i++) { var prev = array[i-1]; var curr = array[i]; array[i-1] = prev + curr; }
let + const
for(let i = 1; i < array.length; i++) { const prev = array[i-1]; const curr = array[i]; array[i-1] = prev + curr; }

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
var only
let + const

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated 26 days ago):

LLMs can make mistakes. Check important info.

**Overview of the Benchmark**

The benchmark is designed to compare the performance of two JavaScript variables: `var` and `let/const`. The benchmark specifically tests how these variables affect the execution speed of a simple loop that updates an array element.

**Script Preparation Code**

The script preparation code creates a large array with 10,000 elements using the `Array.from()` method. This array is used as the input for the benchmark test.

**Html Preparation Code**

There is no HTML preparation code provided in this benchmark definition.

**Benchmark Test Cases**

There are two individual test cases:

1. **"var only"`**: This test case uses the `var` variable declaration and demonstrates how it affects performance.
2. **"let + const"`**: This test case uses the `let` and `const` variable declarations and compares its performance to that of `var`.

**Comparison of Variable Declarations**

The benchmark tests three scenarios:

*   **var only**: The test code uses the `var` variable declaration, which creates a new scope for each iteration of the loop. This can lead to slower performance due to:
    *   **Hoisting**: Variables declared with `var` are "hoisted" to the top of their scope, causing unnecessary computations and potential performance issues.
    *   **Scope creation and destruction**: Each iteration creates a new scope, which adds overhead due to memory allocation and deallocation.

*   **let only**: The test code uses the `let` variable declaration, which also creates a new scope for each iteration. However, it is more efficient than `var` because:
    *   **No hoisting**: Variables declared with `let` are not hoisted, reducing unnecessary computations and potential performance issues.
    *   **No scope creation and destruction**: Since `let` does not create a new scope on every iteration, the overhead of memory allocation and deallocation is reduced.

*   **let + const**: The test code uses both `let` and `const` variable declarations, which provides the benefits of `let` (no hoisting and no scope creation and destruction) while also ensuring that variables are not reassigned.

**Pros and Cons of Each Approach**

*   **var only**:
    *   Pros: None
    *   Cons:
        +   Hoisting can lead to slower performance.
        +   Scope creation and destruction add overhead due to memory allocation and deallocation.

*   **let/const**:
    *   Pros:
        -   No hoisting, reducing unnecessary computations and potential performance issues.
        -   No scope creation and destruction, reducing the overhead of memory allocation and deallocation.
    *   Cons: None notable.

**Library Used**

There is no library explicitly mentioned in this benchmark definition. However, it uses `Array.from()` to create an array, which is a built-in JavaScript method that can be considered as part of the language's standard library.

**Special JS Features or Syntax**

This benchmark does not use any special JavaScript features or syntax other than variable declarations (`var`, `let`, and `const`).

**Alternatives**

Other alternatives for benchmarking JavaScript performance could include:

*   Using a different JavaScript engine, such as V8 (used by Chrome) or SpiderMonkey (used by Firefox).
*   Using a different language or combination of languages to write the test code.
*   Using a profiling tool to identify performance bottlenecks in the application.

Keep in mind that the choice of benchmarking methodology and setup will depend on the specific use case and requirements.

Latest run results:

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

User agent: Mozilla/5.0 (X11; Linux x86_64; rv:94.0) Gecko/20100101 Firefox/94.0

Browser/OS: Firefox 94 on Linux

View result in a separate tab

Test name	Executions per second
var only	10055.5 Ops/sec
let + const	10218.8 Ops/sec

Related benchmarks:

var only vs let + const (version: 0)

Comparing performance of: var only vs let + const

Created: 6 years ago by: Guest

Jump to the latest result

var only

let + const

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated 26 days ago):