discount-promises.js

function nextTick(callback) {
    setTimeout(callback, 0);
}

class Promise {
    constructor(callback) {
        this.state = 'pending';
        this.onSuccess = [];

        callback(value => {
            this.state = 'resolved';
            this.value = value;
            this.onSuccess.forEach(callback => callback(value));
        });
    }

    then(callback) {
        return new Promise(resolve => {
            if (this.state === 'resolved') {
                nextTick(() => resolve(callback(this.value)));
            } else {
                this.onSuccess.push(value => resolve(callback(value)));
            }
        });
    }
}