1 00:00:03,920 --> 00:00:08,675 In this lecture and the exercise that follows, 2 00:00:08,675 --> 00:00:12,000 we'll talk specifically about Callback Hell. 3 00:00:12,000 --> 00:00:15,330 The problem that arises when you write 4 00:00:15,330 --> 00:00:19,980 callbacks in node and especially nested callbacks and node. 5 00:00:19,980 --> 00:00:26,795 We'll also look at promises as one way of addressing the Callback Hell problem. 6 00:00:26,795 --> 00:00:34,340 To give you an idea of what the typical Callback Hell problem will result in, 7 00:00:34,340 --> 00:00:40,115 let's revisit the index.js file from the previous exercise. 8 00:00:40,115 --> 00:00:44,020 As I was talking about it in the previous exercise 9 00:00:44,020 --> 00:00:47,959 you notice that for each of the operations that we perform, 10 00:00:47,959 --> 00:00:53,990 the subsequent operation is included inside the callback function here 11 00:00:53,990 --> 00:01:00,320 and so on until you form a pyramid like structure go again. 12 00:01:00,320 --> 00:01:06,160 So you see this pyramid like structure arising within your code. 13 00:01:06,160 --> 00:01:11,540 Now imagine that you need to perform multiple nested operations like this, 14 00:01:11,540 --> 00:01:13,850 your code will become pretty soon, 15 00:01:13,850 --> 00:01:18,085 very complex, and not so easy to decipher. 16 00:01:18,085 --> 00:01:22,240 Now this is what in the node.js world, 17 00:01:22,240 --> 00:01:25,830 they refer to as Callback Hell. 18 00:01:25,960 --> 00:01:30,845 There have been several ways of mitigating the problem that arises. 19 00:01:30,845 --> 00:01:35,690 Now you can completely avoid this because of the fact 20 00:01:35,690 --> 00:01:38,300 that you have certain operations that need to 21 00:01:38,300 --> 00:01:41,480 complete before the next operation can be initiated, 22 00:01:41,480 --> 00:01:47,765 but we can rearrange the code in a way to mitigate this problem. 23 00:01:47,765 --> 00:01:52,080 Promises being one such way of addressing this. 24 00:01:52,080 --> 00:01:56,015 So to summarize what we have just discussed, 25 00:01:56,015 --> 00:01:59,435 heavily nested callback code, 26 00:01:59,435 --> 00:02:02,990 causes the Callback Hell problem and it 27 00:02:02,990 --> 00:02:07,010 results from our tendency to write programs top-down. 28 00:02:07,010 --> 00:02:11,990 We are still hung up with our sequential way of writing code and 29 00:02:11,990 --> 00:02:17,420 so we see it more convenient to write code top to bottom, 30 00:02:17,420 --> 00:02:21,145 and look at it as if it is executing in that order. 31 00:02:21,145 --> 00:02:24,140 Now we can work around the Callback Hell problem by 32 00:02:24,140 --> 00:02:27,460 not using anonymous functions for the callbacks but instead, 33 00:02:27,460 --> 00:02:30,870 declaring those functions with specific names, 34 00:02:30,870 --> 00:02:35,630 and then avoid the way we write the code as you saw here. 35 00:02:35,630 --> 00:02:41,190 That is one of the approaches that people take to deal with the Callback Hell problem. 36 00:02:41,190 --> 00:02:44,990 There are several other approaches that have been suggested, 37 00:02:44,990 --> 00:02:51,085 links to a couple of articles in this regard are provided in the additional resources. 38 00:02:51,085 --> 00:02:53,720 But in this particular lecture, 39 00:02:53,720 --> 00:02:56,810 I will concentrate on one particular approach that 40 00:02:56,810 --> 00:03:00,275 is used to deal with for the Callback Hell problem, 41 00:03:00,275 --> 00:03:02,370 that is the use of promises. 42 00:03:02,370 --> 00:03:07,345 So we can use promises to tame the Callback Hell problem to quite an extent. 43 00:03:07,345 --> 00:03:12,200 We will look at how promises help us in this regard and in 44 00:03:12,200 --> 00:03:17,180 the exercise we will see how because the NODE-MONGO DB driver already 45 00:03:17,180 --> 00:03:21,665 supports a proxy interface we can leverage that to rewrite our code 46 00:03:21,665 --> 00:03:27,495 to take advantage of promise support in the MONGO DB driver. 47 00:03:27,495 --> 00:03:31,455 Briefly summarizing what a promise is. 48 00:03:31,455 --> 00:03:35,625 A promise is a mechanism that supports asynchronous computation. 49 00:03:35,625 --> 00:03:39,250 So if you have amount of work that needs to be done, 50 00:03:39,250 --> 00:03:43,550 the promise acts as a proxy for a value which is 51 00:03:43,550 --> 00:03:48,100 not known at the moment but the promise is given to you. 52 00:03:48,100 --> 00:03:50,270 But when the value becomes available, 53 00:03:50,270 --> 00:03:53,225 it will be available in the future. 54 00:03:53,225 --> 00:03:56,690 So the promise represents a placeholder for that value. 55 00:03:56,690 --> 00:03:58,795 If the value results correctly, 56 00:03:58,795 --> 00:04:02,240 then your promise results correctly and you can have 57 00:04:02,240 --> 00:04:08,735 a piece of code execute in order to handle the fact that the promise resolved correctly, 58 00:04:08,735 --> 00:04:12,950 if not then you handle the error in that situation. 59 00:04:12,950 --> 00:04:20,060 So, a promise will resolve either into resolve or the rejection of the promise. 60 00:04:20,060 --> 00:04:23,515 A pending promise might either resolve 61 00:04:23,515 --> 00:04:27,105 when the value is correctly obtained so in that case, 62 00:04:27,105 --> 00:04:31,755 it will resolve or what we call as the fulfilling of the promise. 63 00:04:31,755 --> 00:04:33,540 So when the promise is resolved, 64 00:04:33,540 --> 00:04:35,960 then you will have a piece of code that 65 00:04:35,960 --> 00:04:38,760 handles the fact that the promise has been resolved. 66 00:04:38,760 --> 00:04:40,455 If the promise is rejected, 67 00:04:40,455 --> 00:04:43,985 you should also handle that situation 68 00:04:43,985 --> 00:04:48,470 the rejection of the promise correspondingly within your code. 69 00:04:48,470 --> 00:04:52,400 So that is the reason whenever you create a promise, 70 00:04:52,400 --> 00:04:56,680 you only supply the resolve and the reject options from it. 71 00:04:56,680 --> 00:05:02,905 The resolve option is typically handled by the doctor then option for your promise. 72 00:05:02,905 --> 00:05:05,960 So why do we use promises? 73 00:05:05,960 --> 00:05:08,060 Promises are used because it addresses 74 00:05:08,060 --> 00:05:13,130 the callback hell problem to a large extent and promises can be changed. 75 00:05:13,130 --> 00:05:15,860 For example, if you have one promise which in 76 00:05:15,860 --> 00:05:20,690 turn triggers a call to another one which will return a promise. 77 00:05:20,690 --> 00:05:26,525 The handling of the promise can be changed to the handle of the previous promise. 78 00:05:26,525 --> 00:05:28,315 So you can have a bunch of, 79 00:05:28,315 --> 00:05:33,635 then calls that will handle the return of the value. 80 00:05:33,635 --> 00:05:39,930 Now we will see the use of this in the exercise that follows this lecture. 81 00:05:39,930 --> 00:05:42,275 To consume a promise, 82 00:05:42,275 --> 00:05:47,675 you will register an appropriate callback function for 83 00:05:47,675 --> 00:05:50,810 when the consumer of a promise is notified either 84 00:05:50,810 --> 00:05:53,750 of the fulfillment or the rejection of the promise. 85 00:05:53,750 --> 00:05:59,860 So the callbacks are registered through the.then() to the promise. 86 00:05:59,860 --> 00:06:07,040 You will use the.catch() to catch the errors within the return promise. 87 00:06:07,040 --> 00:06:09,845 Now the.then() methods can be chained 88 00:06:09,845 --> 00:06:15,205 together as you will see in the exercise that follows. 89 00:06:15,205 --> 00:06:19,730 As an example, you would normally handle a promise by 90 00:06:19,730 --> 00:06:26,250 chaining the.then() and the.catch() to the promise value.