1
00:00:00,475 --> 00:00:05,283
Now you're going to learn how to create a
promise and get a result out of a promise,

2
00:00:05,283 --> 00:00:07,530
then how to chain promises together.

3
00:00:07,530 --> 00:00:11,809
A promise is always going to be
in one of three possible states,

4
00:00:11,809 --> 00:00:14,408
pending, fulfilled, or rejected.

5
00:00:14,408 --> 00:00:18,202
The pending state is like the waiting
state you're in after being handed

6
00:00:18,202 --> 00:00:18,832
the pager.

7
00:00:18,832 --> 00:00:21,160
It's the default state of a promise.

8
00:00:21,160 --> 00:00:24,956
Fulfilled means that the operation
completed successfully.

9
00:00:24,956 --> 00:00:28,180
And rejected means that
the operation failed.

10
00:00:28,180 --> 00:00:31,264
Working with promises is
a three step sequence.

11
00:00:31,264 --> 00:00:35,429
You create a promise instance
using the Promise() constructor.

12
00:00:35,429 --> 00:00:40,283
You then define what should happen when
the promise is fulfilled or rejected.

13
00:00:40,283 --> 00:00:44,966
Then call one of the methods provided
by the promise API to either consume

14
00:00:44,966 --> 00:00:49,342
the value of a fulfilled promise,
or provide a rejection reason for

15
00:00:49,342 --> 00:00:50,751
a rejected promise.

16
00:00:50,751 --> 00:00:52,591
Let's see how all of this works.

17
00:00:52,591 --> 00:00:56,206
In the project,
open the file promises-breakfast.js.

18
00:00:56,206 --> 00:01:01,021
In this file, let's create a new
promise called breakfast promise

19
00:01:01,021 --> 00:01:03,309
with const breakfastPromise.

20
00:01:03,309 --> 00:01:05,652
We create the promise
using the new keyword,

21
00:01:05,652 --> 00:01:08,432
followed by the Promise()
constructor function.

22
00:01:10,473 --> 00:01:14,822
The Promise() constructor
takes one argument,

23
00:01:14,822 --> 00:01:18,664
a callback with two perimeters,
resolve for

24
00:01:18,664 --> 00:01:24,543
when the promise is fulfilled, and
reject for when it's rejected.

25
00:01:24,543 --> 00:01:30,358
Within the callback, I'll write an async
task using the setTimeout method.

26
00:01:30,358 --> 00:01:38,623
I'll pass it a callback and set a delay
value of 3,000 milliseconds, or 3 seconds.

27
00:01:38,623 --> 00:01:41,602
Our Promise() constructor
function is all set up.

28
00:01:41,602 --> 00:01:46,781
This function is going to return
a promise object that is either pending,

29
00:01:46,781 --> 00:01:48,627
resolved, or rejected.

30
00:01:48,627 --> 00:01:53,137
For example,
if I console.log(breakfastPromise),

31
00:01:53,137 --> 00:01:58,750
then in the terminal run this file
with node promises-breakfast.js,

32
00:01:58,750 --> 00:02:04,925
notice how the console outputs a promise
object, which is currently pending.

33
00:02:04,925 --> 00:02:09,154
And that's correct because pending
is the default state of a promise.

34
00:02:09,154 --> 00:02:13,797
So now, inside the constructor,
we can either call resolve to

35
00:02:13,797 --> 00:02:18,438
return a promise object that is
resolved with a given value, or

36
00:02:18,438 --> 00:02:23,713
reject to return a promise object
that is rejected with a given reason.

37
00:02:23,713 --> 00:02:26,385
Both of these functions
are provided by the constructor.

38
00:02:26,385 --> 00:02:31,052
So first, inside setTimeout,
I'll call resolve.

39
00:02:31,052 --> 00:02:33,749
And resolve does just that, it changes or

40
00:02:33,749 --> 00:02:37,842
resolves the status of the promise
from pending to fulfilled.

41
00:02:37,842 --> 00:02:42,242
We can pass resolve a value which
then becomes the fulfillment value of

42
00:02:42,242 --> 00:02:43,146
the promise.

43
00:02:43,146 --> 00:02:51,518
For example, I'll pass resolve the string
Your order is ready, come and get it!

44
00:02:51,518 --> 00:02:56,556
So when the asynchronous task completes
successfully, it's going to return

45
00:02:56,556 --> 00:03:01,758
a resolved promise object with Your order
is ready, come and get it as its value.

46
00:03:01,758 --> 00:03:04,019
Running the file again in the console,

47
00:03:04,019 --> 00:03:06,629
we see that the promise
is in a pending state.

48
00:03:06,629 --> 00:03:11,888
So now we need to get the value out of the
promise object, or consume the promise.

49
00:03:11,888 --> 00:03:16,846
To do that, the Promise API offers
methods you can call on the promise.

50
00:03:16,846 --> 00:03:21,368
The two you'll likely use
most are then() and catch().

51
00:03:21,368 --> 00:03:26,359
So first,
I'll call then() on our breakfast

52
00:03:26,359 --> 00:03:30,735
promise with breakfastPromise.then.

53
00:03:30,735 --> 00:03:35,669
then() can be called to handle both
fulfilled and rejected promises.

54
00:03:35,669 --> 00:03:40,130
It accepts two functions as its argument,
one for fulfilled promises, and

55
00:03:40,130 --> 00:03:42,510
an optional one for rejected promises.

56
00:03:42,510 --> 00:03:45,949
In this case, I'm only calling it
to handle fulfilled promises, and

57
00:03:45,949 --> 00:03:48,766
later use the catch method
to handle rejected promises.

58
00:03:48,766 --> 00:03:54,053
So to get the resolved value, I'll pass
then() an arrow function that takes

59
00:03:54,053 --> 00:03:59,514
the fulfillment value as a parameter I'll
name val, and logs it to the console.

60
00:04:03,467 --> 00:04:06,383
Now I'll run this file
again in the console,

61
00:04:06,383 --> 00:04:08,694
we see that the promise is pending.

62
00:04:08,694 --> 00:04:13,660
Then after three seconds, it resolves to
the value Your order is ready, come and

63
00:04:13,660 --> 00:04:14,183
get it.

64
00:04:14,183 --> 00:04:21,043
Good, so after the three second delay, the
promise changes from pending to fulfilled.

65
00:04:21,043 --> 00:04:25,561
Then the function passed to
then() is immediately invoked,

66
00:04:25,561 --> 00:04:28,384
which returns the fulfillment value.

67
00:04:28,384 --> 00:04:33,106
In other words,
whatever we pass the resolve method is

68
00:04:33,106 --> 00:04:36,504
available to be used here inside then.

69
00:04:36,504 --> 00:04:39,294
Not all promises are resolved,
so in the next video,

70
00:04:39,294 --> 00:04:43,270
you'll learn how to handle rejected
promises using the catch() method.