Benchmark: JS class instanceof vs key in

Script Preparation code:

const fooKey = Symbol('fooSymbol')

class Foo {
    constructor(value) {
        this[fooKey] = true
        this.value = value
    }
}

let arr;

function prepare() {
    arr = Array.from({length:1000})
    for (let i = 0; i < arr.length; i++) {
        arr[i] = new Foo(i)
    }
}

function runInstanceof() {
    prepare();
    const match = arr.every((foo) => foo instanceof Foo);
    if (!match) throw new Error('fail!');
}

function runKeyIn() {
    prepare();
    const match = arr.every((foo) => fooKey in foo);
    if (!match) throw new Error('fail!');
}

Tests:

instanceof
runInstanceof()
key in
runKeyIn()

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
instanceof
key in

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: 11 months ago)

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.6 Safari/605.1.15

Browser/OS: Safari 18 on Mac OS X 10.15.7

View result in a separate tab

instanceof

274K/s

key in

267K/s

View exact numbers

Test name	Executions per second
✓ instanceof	273,788 Ops/sec
key in	266,796 Ops/sec

Autogenerated LLM Summary (model gpt-4o-mini, generated one year ago):

LLMs can make mistakes. Check important info.

The benchmark defined in the provided JSON compares two different methods for verifying whether an object is an instance of a class or contains a specific property. Specifically, it tests the performance of using the `instanceof` operator versus using the `in` operator (with a key that is a Symbol) in JavaScript.

### Benchmark Options:

1. **`foo instanceof Foo`**:
   - **Description**: The `instanceof` operator checks whether an object is an instance of a specified constructor (in this case, the `Foo` class).
   - **Use Case**: It's commonly used for checking the type of objects and is particularly useful for class instances since it respects the prototype chain.

2. **`fooKey in foo`**:
   - **Description**: The `in` operator checks if a specified property (identified by `fooKey`, which is a Symbol) exists in the object or its prototype chain.
   - **Use Case**: This approach is used to determine if an object has a specific property, regardless of its value.

### Pros and Cons:

**1. Using `instanceof`:**
   - **Pros**:
     - Clear intention; it directly conveys the intent of checking an object's type.
     - Can handle inheritance properly, which is useful in class hierarchies.
   - **Cons**:
     - Performance can vary based on the complexity of the prototype chain and whether the object's constructor chain is extensive.
     - Does not work with primitive types or when checking for properties.

**2. Using `key in`:**
   - **Pros**:
     - More flexible in checking for the presence of properties without concern for the object's constructor.
     - Fast performance for property checking, especially when combined with Symbols (which are unique).
   - **Cons**:
     - Less clear for type checking. It can lead to confusion if the property might exist on the prototype chain.
     - It could yield false positives if you're unsure about property inheritance and if the property exists on other prototypes.

### Library and Syntax:

The benchmark utilizes:
- **Symbol**: A built-in JavaScript feature that creates unique identifiers. It is often used as property keys to avoid name clashes and to create private properties/methods.

In this benchmark, the Symbol is used to create a key (`fooKey`) that is assigned to an object property, making it less susceptible to name conflicts and ensuring that checking for the property is safe and specific.

### Overall Considerations:

This benchmark is useful for software engineers to understand performance trade-offs between two methods for verifying object characteristics in JavaScript. Particularly in high-performance applications, knowing when to use one method over the other can significantly impact efficiency.

### Alternatives:

Apart from `instanceof` and `in`, other alternatives to check types or properties can include:

- **Type Checking (using `typeof`)**: Good for primitive types but not suitable for class instances.
- **Custom Type Guards**: More complex but gives full control over type checking.
- **Object.hasOwnProperty()**: Checks if the property is a direct property of the object, excluding the prototype chain, which can be useful in certain cases.

Understanding these options provides valuable context for choosing the right method based on performance requirements, code clarity, and the structure of the data being dealt with.

JS class instanceof vs key in (version: 1)

Comparing performance of: `instanceof vs key in`

Created: one year ago by: Guest

Go to the latest result

instanceof

key in

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model gpt-4o-mini, generated one year ago):

JS class instanceof vs key in (version: 1)

Comparing performance of: instanceof vs key in

Created: one year ago by: Guest

Go to the latest result

instanceof

key in

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model gpt-4o-mini, generated one year ago):

Related benchmarks

Comparing performance of: `instanceof vs key in`