How to subclass ES6 Promises with customized constructor. 99% of the time, you shouldn't.

promise_subclass.js

class ValueInTime
{
    constructor (value, time)
    {
	this.result = new Promise ((fulfill, reject) => {
	    this.fulfill = fulfill
	    this.reject = reject
	})
	this.value = value
	this.time = time
	this.begin()
    }

    then ()
    {
	return this.result.then.apply(this.result, arguments)
    }

    begin ()
    {
	setTimeout(() => this.fulfill(this.value), this.time * 1000)
    }
}

async function test ()
{
    for (let n=0; n<3; ++n)
	console.log(await new ValueInTime(n, 1))
}

test()