Promises

Frontend Developer

image/svg+xml

Aries

ES6

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

Promises let you start with code like this:

...and refactor it into this:

require("popsicle")
    .get("http://www.google.com")
    .then(function(response) {
        console.log(response.body);
    })

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

Pending

fULFILLED

REJECTED

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")
    }
})
promiseToClean.then(function(value) {
    console.log("the room is " + value)
}).catch(function() {
    console.log('the room is' + value)
})
Promise.catch() 

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')
    })
};
cleanRoom().then(function(result) {
    return removeGarbage(result);
}).then(function(result) {
    return winIcecream(result);
}).then(function(result) {
    console.log('finished!\n' + result);
})
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! 

Promises

By erica mitchell

Promises

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