promyse.js

class Promyse {
    constructor(userFunction) {
        this.state = 'pending';
        this.thenCallback = null;
        this.errorCallback = null;

        userFunction(this._resolve.bind(this), this._reject.bind(this));
    }

    _resolve(data) {
        this.state = 'fulfilled';
        this.data = data;
        this.thenCallback(data);
    }

    _reject(data) {
        this.state = 'rejected';
        this.data = data;
        this.errorCallback(data);
    }

    then(fn) {
        if(this.state === 'fulfilled') {
            fn(this.data);
            return this;
        }
        this.thenCallback = fn;
        return this;
    }

    catch(fn) {
        if(this.state === 'rejected') {
            fn(this.data);
            return this;
        }
        this.errorCallback = fn;
        return this;
    }
}



function createUser(name) {
    return new Promyse((resolve, reject) => {
        setTimeout(() => {
            if(name === 'Adam') {
                return reject(new Error('Such greatness cannot be recreated!'));
            }
            return resolve('User ' + name + ' created');
        }, 3000);
    });
}

createUser('Kasper')
    .then((response) => {
        console.log(response);
    })
    .catch((err) => {
        console.error('Error creating Kasper', err);
    });

createUser('Adam')
    .then((response) => {
        console.log(response);
    })
    .catch((err) => {
        console.error('Error creating Adam', err);
    });