Simple example of a Promise pattern

promise.js

/**
 * promise object
 * Explanation: When creating a promise object, it is supposed to be in a pending state.
 * If a certain process succeeded, we call resolve() callback and it is assumed resolved,
 * also called fulfilled. Meaning, you fulfilled your promise :)
 * If the process failed, than the promise is rejected.
 */
let promise = new Promise((resolve, reject) => {
    // We're now in a pending state
    User.find({}, (user, err) => {  // <-- this an async work
        // on failure (from pending --> rejected)
        if(err) return reject(new Error('error message');
                  
        // on success
        resolve(user); // from pending --> resolved
    }
});
    
/**
 * 
 */
promise.then(user => {
    // we got a successfull object, send it back to the user or
    // something like that.
})
.catch(ex => {
    // send or throw an error
});