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!');

}