Promises

ES6

require("request")
    .get("http://www.google.com", function(err, response) {
          console.log(response.body)
    })

Promises let you start with code like this:

More complicated example with callbacks

require("request")
    .get("http://www.google.com", function(err, response) {
         if (err) {
             console.error(err);
          } else {
              require("fs")
              .writeFile("google.html", response.body, function(err) {
                  if (err) {
                      console.error(err);
                  } else {
                      console.log("wrote file");
                  }
              })
          }
    })

This is the promise version

require("popsicle").get("http://www.google.com")
    .then(function(response) {
        return require("fs-promise").writeFile("google.html", response.body);
    })
    .then(function() {
        console.log("wrote file");
    })
    .catch(function(err) {
        console.log(err);
    })

promises are...

A Design Pattern

Native in JavaScript

Available in libraries

Promise State

A promise can be in one of 3 states:

let promiseToClean = new Promise(function( resolve, reject) { 
 
    let isClean = true;

    if (isClean) {
        resolve('the room is Clean')
    } else {
        reject('the room is not clean')
    }
})

creating a promise

Promise.then() 

let promiseToClean = new Promise(function(resolve, reject) {

    let isClean = true;

    if (isClean) {
        resolve("is clean")
    } else {
        reject("is not clean")
    }
})

more complex example

let cleanRoom = function() {
    return new Promise(function(resolve, reject) {
        resolve('Cleaned the Room\n')
    })
};

let removeGarbage = function(message) {
    return new Promise(function(resolve, reject) {
        resolve(message + 'Removed the garbage\n')
    })
};

let winIcecream = function(message) {
    return new Promise(function(resolve, reject) {
        resolve(message + 'Won Ice Cream\n')
    })
};

Promise.all([])

Promise.all([cleanRoom(), removeGarbage(), winIcecream()]).then(function() {
    console.log('all finished')
})

Promise.race([])

Promise.race([cleanRoom(), removeGarbage(), winIcecream()]).then(function() {
    console.log('one of them is finished')
})

callback hell? 

function(request, response) {
    var user, notebook;

    User.get(request.user)
    .then(function(aUser) {
        user = aUser;
        return Notebook.get(user.notebook);
    })
    .then(function(aNotebook) {
        notebook = aNotebook;
        return doSomethingAsync(user, notebook);
    })
    .then(function(result) {
        response.send(result)
    })
    .catch(function(err) {
        response.send(err)
    })
}

thanks!