What is a Promise?

[MUSIC] 0:00 Promises in JavaScript offer a more elegant and human readable 0:04 way to manage asynchronous code without having to write too many callbacks, or 0:07 spend extra time figuring out what the program should do. 0:12 Promises also give you more control over your asynch programs. 0:16 What is a promise? 0:20 Well a promise represents the eventual completion of a asynchronous operation. 0:21 In other words, 0:26 a value that may not be available yet, like data requested from a server. 0:27 So let's spend a little bit of time unpacking what all this means 0:31 by first looking at the differences between callbacks and promises. 0:34 Let's say that, 0:39 instead of making breakfast yourself, you're in the mood to order breakfast. 0:39 There are two ways you might order breakfast. 0:43 You can order by phone and have it delivered, or 0:45 you can order at the restaurant and pick it up when ready. 0:48 In the first scenario, you call your favorite breakfast spot and 0:51 place your order, providing them your phone number and address. 0:55 The order taker says okay, thanks. 0:58 I'll put in your order now and 1:00 call you back to let you know when the order is ready and on its way. 1:02 And that's it. 1:04 At this point, you can go on and 1:05 do other things while breakfast gets there, that's convenient. 1:07 But you're also left waiting for a call back from the restaurant. 1:10 You really don't have control over when they're going to call you. 1:14 You received nothing in exchange. 1:17 For example, a ticket number to confirm your order. 1:19 You just wait for them to call and 1:21 deliver your food at around the time they said they would. 1:23 In this scenario, you've handed control over whether or 1:26 not you will have breakfast to the person taking your order. 1:29 You expect it to happen, but maybe they wrote down the wrong phone number, and 1:32 they can't call back or they call, but your food never makes it to you. 1:35 Perhaps they even deliver the wrong order. 1:39 Now, most of these problems could be eventually rectified, but 1:41 it's a messy process. 1:44 A process which hands the control back to you when, and only when, 1:45 breakfast is delivered successfully. 1:50 This is similar to working with callback functions. 1:52 You're in some ways handing over control of your function to a callback 1:55 expecting something else to be called and executed. 1:59 Like callbacks, the process could be more susceptible to errors and 2:01 harder to debug if something goes wrong. 2:06 In the second scenario, you go to the breakfast place, put in your order and 2:08 pay for it. 2:12 The cashier then hands you something back in return, a receipt with an order number 2:13 and a pager that buzzes to let you know that your order is ready. 2:17 At that point, you may go off and 2:21 do other things while you wait, like grab a coffee next door or catch up on email. 2:22 You're still waiting for the pager to buzz. 2:27 But by just having the pager in hand, you now have more control over whether or 2:29 not you will have breakfast. 2:34 The pager represents a promise that something is going to happen as a result 2:35 of ordering breakfast. 2:40 With that being said, the promise is pending because you're waiting, so 2:42 one of two things could happen. 2:45 The pager buzzes, your order becomes available to you, 2:47 so you pick it up and breakfast is fulfilled, or the pager buzzes but 2:50 not because your order is ready. 2:55 Instead, it's to inform you that your food cannot be made and 2:57 the reason why your order was in some way rejected. 3:00 This second scenario happens to be more like how promises in JavaScript work. 3:03 A promise, like a pager, represents a value that can be 3:07 handled at some point later, like picking up your order. 3:11 While a callback may leave us with some uncertainties about that value, 3:14 a promise will promise or guarantee a future value and nothing can change it, 3:18 regardless of whether or not it comes back fulfilled or rejected. 3:23 Up next, you will learn how to create a promise, then we'll continue with 3:28 our people in space project by converting the callback functions to promises. 3:32