promises-draft.md

write it out

my name's sarah

• work at venmo on web team for almost a year
• been an engineer a little longer than that (flatiron school)

i essentially learned js at venmo

• applause, ooh-ing and ah-ing
• lots of things made sense/i could pick up w/ relative ease -- iterating over data, ajax calls -- but others were tougher

and promises really didn't make sense

• one of the things that didn't make sense when learning js were promises
  • what is a promise? if i were to hold this promise in my hand like __________, what would it have to tell me? did it have an answer to the task i'd asked it to perform?
  • show some code here, maybe a venmo example

and -- let's be clear -- i know what a promise is

• in the real world

  • according to google:

  a declaration or assurance that one will do a particular thing or that a particular thing will happen.

no really, i know

• we make promises all the time
• just the other day, i promised my mom i would call her more often
• i promised my roommates i'd take out the trash

what's a promise in js?

• in the js world

  • according to an article:

  a promise represents the eventual result of an asynchronous operation

er, what's an asynchronous operation

• synchronous code
  • a bunch of statements in a sequence -- executed one after the other
  • each statement has to wait for the previous one to finish executing

  console.log('one');
  console.log('two');
  console.log('three');

  • metaphor it's like when...
• asynchronous
  • takes statements outside of main program flow

  console.log('one');
  setTimeout(function() {
    console.log('two');
  }, 0);
  console.log('three');

  • metaphor it's like when...

what does this have to do with js

• js is single-threaded and synchronous
  • two bits of script can't run at the same time, but rather must run one after the other
  • in browsers, js shares a thread w/ lots of other stuff
    • generall shares a queue w/ painting, updating styles, handling user actions, etc
    • activity in one of these things delays another
  • metaphor it's like when...
  • (what can make js async is that it has various callback mechanisms)

ok cool, let's revisit what js promise is

• the definition

  a promise represents the eventual result of an asynchronous operation

• so given what we now know about how js is executed...

why would we use promises

• provides a simpler alternative for async operations
• allows for other things to execute while another job is being done

how do promises work

• a promise can be
  • pending -- outcome tbd
  • fulfilled -- completed, and the promise has a value
  • rejected -- failed, has a reason why it failed
  • settled -- has fulfilled/rejected

let's look at a promise

var promise = new Promise(function(resolve, reject) {
  // do a thing, possibly async, then…

  if (/* everything turned out fine */) {
    resolve("Stuff worked!");
  }
  else {
    reject(Error("It broke"));
  }
});
promise.then(function(result) {
  console.log(result); // "Stuff worked!"
}, function(err) {
  console.log(err); // Error: "It broke"
});

how do we use them

• as we saw earlier, we can use them straight out of js
• there are also lots of libraries available
• q, bluebird, when, winjs, rsvpjs, jquery
• personal favs: q and bluebird

let's look at an example of a promise using a library