Benchmark: const vs let vs var

Tests:

var
var g = { e: [] } g.o = function(x) { g.e.push(...[1,2,3]) } g.o()
let
var g = { e: [] } g.o = function(x) { g.e.push(...[1,2,3]) } g.o()
const
const g = { e: [] } g.o = function(x) { g.e.push(...[1,2,3]) } g.o()

Historical Performance Data aggregated from 294 community runs

Desktop Results:

Chrome 191 runs

var

17.1M/s

const

8.3M/s

let

7.8M/s

Fastest: var (2.2x faster)

View exact numbers

✓ var: 17,067,164 ops/sec
• const: 8,311,738 ops/sec
• let: 7,813,376 ops/sec

Firefox 64 runs

const

17.9M/s

var

17.7M/s

let

17.7M/s

View exact numbers

✓ const: 17,936,660 ops/sec
• var: 17,736,856 ops/sec
• let: 17,659,318 ops/sec

Safari 5 runs

var

30.8M/s

let

28.0M/s

const

27.6M/s

Fastest: var (1.1x faster)

View exact numbers

✓ var: 30,847,076 ops/sec
• let: 27,993,298 ops/sec
• const: 27,563,200 ops/sec

Mobile Results:

Chrome 28 runs

var

9.3M/s

let

4.7M/s

const

4.2M/s

Fastest: var (2.2x faster)

View exact numbers

✓ var: 9,288,679 ops/sec
• let: 4,690,266 ops/sec
• const: 4,245,050 ops/sec

Firefox 6 runs

let

12.6M/s

const

12.5M/s

var

12.4M/s

View exact numbers

✓ let: 12,565,872 ops/sec
• const: 12,499,552 ops/sec
• var: 12,419,634 ops/sec

Results show median performance across all community submissions. Run this benchmark yourself to verify on your device.

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 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/140.0.0.0 Mobile Safari/537.36

Browser/OS: Chrome Mobile 140 on Android

View result in a separate tab

let

6.5M/s

const

6.4M/s

var

6.3M/s

View exact numbers

Test name	Executions per second
✓ let	6,483,140 Ops/sec
const	6,368,289 Ops/sec
var	6,321,396 Ops/sec

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

LLMs can make mistakes. Check important info.

I'll explain what's tested in the provided JSON benchmark.

The test measures the performance difference between three JavaScript variable declarations: `const`, `let`, and `var`. The script is designed to push an array of numbers onto a nested object using two different methods: assigning directly to the property (`g.e`) versus calling a function that modifies the same property (`g.o()`).

**Options compared:**

1. `const` (constant): Declares a variable that cannot be reassigned.
2. `let`: Declares a variable that can be reassigned, but not within its own block scope (i.e., inside an `if` or `loop` statement).
3. `var`: Declares a variable that can be reassigned anywhere in the function.

**Pros and cons of each approach:**

* `const`:
	+ Pros: Memory efficiency, because the variable cannot be changed, which reduces the likelihood of unintended side effects.
	+ Cons: Cannot be reassigned; if the value is needed to be modified, a new constant should be created.
* `let`:
	+ Pros: More efficient than `var`, since it redeclares the variable's scope and creates a new context.
	+ Cons: Not suitable for functions or loops where reassignment might occur.
* `var`:
	+ Pros: Can be used in any context, but it can lead to unintended side effects due to global scope.
	+ Cons: Memory inefficiency, as variables are stored on the global scope, and their values can affect other parts of the code.

**Library usage:** None

There is no external library used in this benchmark.

**Special JS features/syntax:** None mentioned

There's no special JavaScript feature or syntax being tested in this benchmark.

Now, let's discuss some alternative approaches to testing performance differences between these variable declarations:

1. **Simple loop**: Create a simple loop that assigns and reassigns values using each variable declaration.
2. **Object iteration**: Create an object with nested properties and iterate over it using each variable declaration.
3. **Array manipulation**: Create an array of numbers and use `map()`, `filter()`, or other methods to modify its contents, comparing the performance differences between `const`, `let`, and `var`.
4. **Nested function calls**: Create a nested function that modifies variables declared with each keyword.

These alternatives can provide more comprehensive insights into the performance characteristics of each variable declaration.

const vs let vs var (version: 0) Benchmark.js

Comparing performance of: `var vs let vs const`

Created: 7 years ago by: Guest

Go to the latest result

var

let

const

Historical Performance Data aggregated from 294 community runs

Desktop Results:

Chrome 191 runs

Firefox 64 runs

Safari 5 runs

Mobile Results:

Chrome 28 runs

Firefox 6 runs

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

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

const vs let vs var (version: 0) Benchmark.js

Comparing performance of: var vs let vs const

Created: 7 years ago by: Guest

Go to the latest result

var

let

const

Historical Performance Data aggregated from 294 community runs

Desktop Results:

Chrome 191 runs

Firefox 64 runs

Safari 5 runs

Mobile Results:

Chrome 28 runs

Firefox 6 runs

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

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

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Comparing performance of: `var vs let vs const`