Run results for: JS class instanceof vs key in

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/133.0.0.0 Safari/537.36

Browser: Chrome 133

Operating system: Mac OS X 10.15.7

Device Platform: Desktop

Date tested: one year ago

Test name	Executions per second
instanceof	39211.1 Ops/sec
key in	39992.2 Ops/sec

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()