Benchmark: JS Performance (const vs let vs var)

Script Preparation code:

const maxObjectSize = 10
const maxRuns = 1000000;

Tests:

CONST <> CONSTRUCTION
for (let i = 0; i < maxRuns; i++) { const x = i; }
CONST <> UPDATE
const x = {v: 0} for (let i = 0; i < maxRuns; i++) { x.v = 1; }
CONST <> DELETE
const x = {} for (let i = 0; i < maxObjectSize; i++) { x[`key_{i}`] = 1 } for (let i = 0; i < maxObjectSize; i++) { delete x[`key_{i}`] }
CONST <> ACCESS
const x = {} for (let i = 0; i < maxObjectSize; i++) { x[`key_{i}`] = 1 } for (let i = 0; i < maxObjectSize; i++) { console.log(x[`key_{i}`]); }
LET <> CONSTRUCTION
for (let i = 0; i < maxRuns; i++) { let x = i; }
LET <> UPDATE
let x = {v: 0} for (let i = 0; i < maxRuns; i++) { x.v = 1; }
LET <> DELETE
let x = {} for (let i = 0; i < maxObjectSize; i++) { x[`key_{i}`] = 1 } for (let i = 0; i < maxObjectSize; i++) { delete x[`key_{i}`] }
LET <> ACCESS
let x = {} for (let i = 0; i < maxObjectSize; i++) { x[`key_{i}`] = 1 } for (let i = 0; i < maxObjectSize; i++) { console.log(x[`key_{i}`]); }
VAR <> ASSIGNMENT
for (let i = 0; i < maxRuns; i++) { var x = i; }
VAR <> UPDATE
var x = {v: 0} for (let i = 0; i < maxRuns; i++) { x.v = 1; }
VAR <> ACCESS
var x = {} for (let i = 0; i < maxObjectSize; i++) { x[`key_{i}`] = 1 } for (let i = 0; i < maxObjectSize; i++) { console.log(x[`key_{i}`]); }
VAR <> DELETE
var x = {} for (let i = 0; i < maxObjectSize; i++) { x[`key_{i}`] = 1 } for (let i = 0; i < maxObjectSize; i++) { delete x[`key_{i}`] }

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
CONST <> CONSTRUCTION
CONST <> UPDATE
CONST <> DELETE
CONST <> ACCESS
LET <> CONSTRUCTION
LET <> UPDATE
LET <> DELETE
LET <> ACCESS
VAR <> ASSIGNMENT
VAR <> UPDATE
VAR <> ACCESS
VAR <> DELETE

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: 3 months ago)

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/109.0.0.0 Safari/537.36

Browser/OS: Chrome 109 on Windows

View result in a separate tab

Test name	Executions per second
CONST <> CONSTRUCTION	852.1 Ops/sec
CONST <> UPDATE	854.1 Ops/sec
CONST <> DELETE	2789804.0 Ops/sec
CONST <> ACCESS	33172.1 Ops/sec
LET <> CONSTRUCTION	851.8 Ops/sec
LET <> UPDATE	847.2 Ops/sec
LET <> DELETE	2768529.0 Ops/sec
LET <> ACCESS	34612.8 Ops/sec
VAR <> ASSIGNMENT	849.5 Ops/sec
VAR <> UPDATE	852.1 Ops/sec
VAR <> ACCESS	34232.7 Ops/sec
VAR <> DELETE	2778016.0 Ops/sec

Autogenerated LLM Summary (model gpt-4o-mini, generated 3 months ago):

LLMs can make mistakes. Check important info.

The benchmark defined in the provided JSON tests the performance differences between three variable declaration keywords in JavaScript: `const`, `let`, and `var`. It focuses on how these keywords affect performance in various scenarios, examining their behavior in terms of creation, updating, accessing, and deleting properties or values.

### Benchmark Options Compared

1. **CONST vs LET vs VAR in Object Construction:**
   - **Test Cases:**
     - `CONST <> CONSTRUCTION`
     - `LET <> CONSTRUCTION`
     - `VAR <> ASSIGNMENT`
   - **Description:**
     Each test case evaluates how quickly each keyword allows for the creation of variables. `const` is used for variables that do not change; `let` is for block-scoped variables, and `var` allows for function-scoped variables.

2. **CONST vs LET vs VAR in Object Updates:**
   - **Test Cases:**
     - `CONST <> UPDATE`
     - `LET <> UPDATE`
     - `VAR <> UPDATE`
   - **Description:**
     These cases measure how efficiently each keyword handles updates to an object's property, where a constant reference (via `const`) does not change, but the property itself can.

3. **CONST vs LET vs VAR in Object Access:**
   - **Test Cases:**
     - `CONST <> ACCESS`
     - `LET <> ACCESS`
     - `VAR <> ACCESS`
   - **Description:**
     This measures the speed of accessing properties of objects that are created with each declaration style.

4. **CONST vs LET vs VAR in Object Deletions:**
   - **Test Cases:**
     - `CONST <> DELETE`
     - `LET <> DELETE`
     - `VAR <> DELETE`
   - **Description:**
     This examines the performance of deleting properties from objects across the three variable types.

### Pros/Cons of Each Approach

- **`const`:**
  - **Pros:** Safer coding practices since it creates immutable bindings. Prevents re-assignment errors, which can reduce bugs.
  - **Cons:** Cannot be re-assigned. While the object reference is constant, nested properties can still be altered, which may lead to confusion.

- **`let`:**
  - **Pros:** Allows variables to be re-assigned while maintaining block scope, which can help in iterative constructs, such as loops, providing more control over variable visibility.
  - **Cons:** Increases the risk of variable hoisting within its block; developers must be cautious about understanding their lifetimes.

- **`var`:**
  - **Pros:** Has the widest scope of relevance—function scoped, allowing for more flexibility in usage in older code bases where its behavior is better understood.
  - **Cons:** Can lead to **hoisting** issues and more complex bugs due to its function scoping, often resulting in less predictable variable behavior.

### Other Considerations

- **Performance:** The results of the benchmark display the varying execution speed for different operations on these variable types.
  
- **Browser/Platform Variation:** The benchmark results are contextualized within a specific browser scenario (Chrome 109 on Windows). Performance can vary significantly across different browsers or devices due to differences in JavaScript engines.

### Alternative Approaches

Other common practices in JavaScript for managing variable scope and performance include:

- Using function scope effectively with `var`, especially in cases where block scope (as created by `let` and `const`) is not required.
- Functional programming techniques can also minimize reliance on mutable state by using functions that do not modify external variables (pure functions).
- Implementing JavaScript Modules (e.g., ES6 module imports/exports) to better manage scope can provide isolation, similar to `let` and `const`.
- Analyzing data with libraries like Lodash or Immutable.js, which could perform better for specific data manipulation tasks by encouraging immutability and functional programming patterns.

By understanding these distinctions and operational contexts, engineers can select the most appropriate variable declaration based on performance characteristics and code semantics.

Related benchmarks:

JS Performance (const vs let vs var) (version: 1)

Comparing performance of: CONST <> CONSTRUCTION vs CONST <> UPDATE vs CONST <> DELETE vs CONST <> ACCESS vs LET <> CONSTRUCTION vs LET <> UPDATE vs LET <> DELETE vs LET <> ACCESS vs VAR <> ASSIGNMENT vs VAR <> UPDATE vs VAR <> ACCESS vs VAR <> DELETE

Created: 3 months ago by: Registered User

Jump to the latest result

CONST <> CONSTRUCTION

CONST <> UPDATE

CONST <> DELETE

CONST <> ACCESS

LET <> CONSTRUCTION

LET <> UPDATE

LET <> DELETE

LET <> ACCESS

VAR <> ASSIGNMENT

VAR <> UPDATE

VAR <> ACCESS

VAR <> DELETE

Suite status: <idle, ready to run>

Experimental features:

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model gpt-4o-mini, generated 3 months ago):