Benchmark: javascript var vs let vs const

Tests:

var
(function () { var arr = [55, 21, 807, 41, 56, 66, 202]; var num = 4; var str = "Lorem ipsum"; var bool = true; var nil = null; console.log(arr, num, str, bool, nil); })();
let
(function () { let arr = [55, 21, 807, 41, 56, 66, 202]; let num = 4; let str = "Lorem ipsum"; let bool = true; let nil = null; console.log(arr, num, str, bool, nil); })();
const
(function () { const arr = [55, 21, 807, 41, 56, 66, 202]; const num = 4; const str = "Lorem ipsum"; const bool = true; const nil = null; console.log(arr, num, str, bool, nil); })();

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
var
let
const

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: 7 months ago)

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/122.0.0.0 YaBrowser/24.4.0.0 Safari/537.36

Browser/OS: Yandex Browser 24 on Windows

View result in a separate tab

Test name	Executions per second
var	104804.0 Ops/sec
let	102573.1 Ops/sec
const	101666.7 Ops/sec

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

LLMs can make mistakes. Check important info.

Let's break down the provided benchmark and explain what's being tested, compared, and the pros/cons of different approaches.

**Benchmark Definition:**

The benchmark is testing the performance difference between three JavaScript variable declarations: `var`, `let`, and `const`.

**Script Preparation Code:**

The script preparation code is empty, which means that the test case only runs a simple JavaScript snippet. In this case, the snippet creates an array `arr` with some values, declares four variables (`num`, `str`, `bool`, and `nil`) and logs their values to the console using `console.log`.

**Individual Test Cases:**

There are three test cases:

1. `var`
2. `let`
3. `const`

Each test case has the same script preparation code, but with a different variable declaration.

**What's Being Tested?**

The benchmark is testing the performance difference between the three variable declarations in terms of execution time. The test case measures how many times the JavaScript engine can execute the script for each variable declaration per second.

**Comparison:**

Here's a brief comparison of `var`, `let`, and `const`:

*   **`var`**: In older versions of JavaScript, `var` declared variables that were "hoisted" to the top of their scope. This meant that even though the variable was assigned a value later in the code, it could be accessed before its assignment. `var` is now considered legacy syntax and should not be used for new code.
*   **`let`**: Introduced in ECMAScript 2015 (ES6), `let` declared block-scoped variables that were not hoisted like `var`. This means that a variable could only be accessed after it was declared. `let` is now the recommended way to declare variables in modern JavaScript.
*   **`const`**: Also introduced in ECMAScript 2015 (ES6), `const` declared constants that also could not be hoisted like `var`. Unlike `let`, `const` cannot be reassigned once it's set.

**Pros and Cons:**

Here are some pros and cons of each variable declaration:

*   **`var`**:
    *   Pros: Hoisting can sometimes make code easier to read.
    *   Cons: Variables can be accessed before they're assigned, which can lead to unexpected behavior.
*   **`let`**: This is now the recommended way to declare variables in modern JavaScript. It's not hoisted, so you need to access a variable after it's declared.
    *   Pros: Block-scoped, prevents unexpected behavior when accessing variables before they're assigned.
    *   Cons: Can be slower than `var` because it doesn't get optimized like hoisted `var`.
*   **`const`**: This is also not hoisted. It's the recommended way to declare constants in modern JavaScript.
    *   Pros: Prevents unexpected behavior when assigning a new value to a variable that's already declared as constant, prevents reassignment of variables that are intended to be immutable.
    *   Cons: Variables cannot be changed once they're assigned.

**Other Considerations:**

1.  **Memory Allocation**: When you declare `var`, the JavaScript engine allocates memory for the variable right away, even if it's not initialized. With `let` and `const`, the memory is allocated when the variable is actually assigned a value.
2.  **Performance**: In general, modern JavaScript engines optimize `var` variables better than `let` and `const`. However, since `let` and `const` are now the recommended way to declare variables in modern JavaScript, it's a good idea to use them for most cases.
3.  **Scope**: The scope of a variable is determined by its declaration statement. If you declare a variable with `var`, it has function scope, meaning it can be accessed from anywhere within the function where it's declared. With `let` and `const`, variables have block scope, which means they're only accessible within the block where they're declared.

**Alternative Options:**

Here are some alternative options to test:

*   **Arrow functions**: You could also compare the performance of arrow functions (introduced in ECMAScript 2015) with regular function declarations.
*   **Object literals**: Comparing the execution speed of object literal expressions versus dot notation (or bracket notation).
*   **Array methods**: Comparing the execution time of different array methods like `forEach`, `map`, and `reduce`.

These alternatives can provide additional insights into the performance characteristics of various JavaScript features.

In summary, the benchmark is testing the performance difference between three variable declarations (`var`, `let`, and `const`) in terms of execution time. The results show that `let` is generally faster than `var`, while `const` is slightly slower due to its reassignment prevention mechanism.

Related benchmarks:

javascript var vs let vs const (version: 0)

Comparing performance of: var vs let vs const

Created: 4 years ago by: Registered User

Jump to the latest result

var

let

const

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

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