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- In this section, we're
going to dig a bit deeper

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into reactive extensions for JavaScript.

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In particular, we're going to see

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how to create an observable

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which emits not just one value

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but a stream of multiple values.

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And then we'll see how we
can test the observable

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in our test code.

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We can receive multiple
values which were emitted

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and we'll also see how to
test for errors as well.

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An observable can emit errors
as well as proper results.

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So we'd need to test for that.

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Okay. If you are interested
in following along,

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this is the folder where
the demos are located

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for this section testing RxJS techniques.

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Okay. So we're gonna look at
three different functions.

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Each will return an observable

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and we'll see how to test
the observable in each case.

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So the first function emit stream one.

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So what I do is there's a form function

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in the reactive extensions library,

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the form function you give it an array

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and it'll create an observable,
which emits those values.

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Okay? So this observable
will emit the value a hundred

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then 200, 300, 400, 500.

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When it's finished, it'll
emit a complete signal to say

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no more data.

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So if you wanted to test that,

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if you wanted to test that
the observable returns

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the correct data, have a look at this.

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This is my test for the
emit stream one function,

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a test in that function there.

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So I call the function.

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It returns an observable.

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That observable is gonna
pump out five values,

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separated by a hundred.

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Okay? So we would expect

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the first value to be
emitted to be a hundred.

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We would expect the second value to be

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200 and then 300 and so on.

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So I've kind of got a little trick here.

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What I've done is I've
set up a, a help available

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which is the value that
I expect to receive next.

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So I subscribe to my observable.

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When I call that function, it
returns an observable object.

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I subscribe to it and I wait
for the next value to arrive.

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Now, the first value that comes
in it should be a hundred.

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So I check that value to be a hundred,

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expected is the current
value of my local variable.

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Okay? So is the value emitted a hundred?

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It should be.

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And then being a bit crafty,

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I then increment my local variable here

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to be 200, ready to test the next value.

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So when the next value is emitted,

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we test that that value is 200

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and then we increment our
expected value to be 300.

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Okay? So the next value that
comes in is that value, 300.

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So this value here in
this hard coded example,

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where I know that the values are separated

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by a hundred each time,

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I can kind of keep track
of that in my code here

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to check that the value that I've got

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is the correct value
and then a hundred more

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for the next value and then a hundred more

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for the next value.

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And so on.

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When it's finished emitting the value 500,

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my observable will then
emit the complete signal

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which I have to handle here

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because you have to,
at some point, tell JS

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there aren't any more values to come.

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So when the complete signal
comes from the observable,

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remember when you
subscribe to an observable,

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it'll give you the next value each time

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but it'll also tell
you when it's finished.

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So when it's finished, we
have to tell JS we're done

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and call in the JS
callback function to say,

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we can move on now, that's
the end of that test.

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There will be no more values
emitted from that observable.

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Okay. Now the next slide,
a bit more realistic

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where we don't necessarily know exactly

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what the values are going to be.

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So if we have a look at this,

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we have a function called emit stream two.

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It's going to create an observable

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which emits a specified number of values.

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So let's say I pass the value
five into this function.

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It'll create an observable
based on this array,

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this array in JavaScript, there's
an array dot form function

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equips an array of the specified length

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and it generates each value via a lambda.

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So if you have a length of five,

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it'll create an array of
five elements and it'll call

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this function to generate
each of those values.

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Basically this generates a number

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between zero and a hundred.

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That's the basic idea.

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So that's what's gonna be generated.

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When I call this function it'll
create an observable object

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and that observable object
will omit the values

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in this array.

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Each value, there will be
five of them, let's say.

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Each value will be or should
be between zero and a hundred.

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So let's see how we
can test that function.

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So to test emit stream
two, call that function

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with the value five.

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So pass the value five in here,

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it creates an array of five elements,

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generates five random numbers
between zero and a hundred

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and emits those values in an observable.

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So it returns an observable object.

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Grab that observable
object and subscribe to it.

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Wait for the next value to arrive.

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That should be a value
between zero and a hundred.

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Okay? So the value that's emitted here,

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according to this business rule,

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it should be a number
between zero and a hundred.

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Check that the value emitted
is in the specified range

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and then wait for the next value

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and the next value.

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That will go around five times,

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that'll be called five times.

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And eventually, you know, when
the observable has finished

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emitting all the values from the array,

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it'll emit the complete signal,

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handle that and tell JS
that's the end of this test.

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One last thing to think about.

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Observables can also emit errors,

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so let's check for that.

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Have a look at this.

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This is my third function.

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So it takes a parameter, but
it does some error check in.

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If the parameter here is like,

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it's meant to be a length of an array,

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but if it's zero or
negative, that's no good.

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So in that case, this is how
you can create an observable

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which emits an error.

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You say, rxjs throw error.

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It creates an observable and
observable raises this error.

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Okay? It basically
triggers an error callback

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with that message.

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So let's see when I call this function

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with a negative value or zero value,

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I can test but it does
indeed throw an error.

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Let's see how to do that.

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Testing that end observable
did raise an error.

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So call the function with
an invalid parameter.

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That should all be in well

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create an observable,
which emits an error.

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Let's subscribe to the observable

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and let's see that it did
actually throw an error.

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Yes, it did.

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What was the error message?

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When an error is emitted,

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the error has a message property.

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Verify that the message
'bad length' is correct.

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And then we can say to JS, we're done.

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Now, if you wanna run any of these tests,

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obviously you need to have the
reactive extensions library

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installed before you can
actually run the tests.

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So let's just take a
quick look at the code.

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I've opened up the code folder

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for testing reactive extension techniques.

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And what I wanted to pick
out is the package.json.

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Remember that you have to include

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the reactive extensions
library in your dependency.

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Otherwise it'll just, you
know, it won't work at all.

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So you need to make sure
that your package json

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includes reactive
extensions for JavaScript

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and then you also need
to do an NPM install.

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Okay? So that'll install the
reactive extensions library

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in your local folder.

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Once you've done that, once
you've installed the local

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once you've installed reactive extensions

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in the local folder, then
you can run the test.

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So I had three functions
that were being tested,

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a function that emitted
in array, a hundred,

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200, 300, 400, 500.

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The second function that emitted numbers

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between zero and 100 five times.

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And the third test, which emitted errors.

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Okay? So they've all worked nicely.

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So there we go.

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It's quite important to know
how to test reactive extensions

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when you are writing in
realistic JavaScript, you will,

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you know, it's not something
that you do all the time,

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but you will need occasionally
to have an observable

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typically from a web socket or a server.

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And you have to verify
that your observable is

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emitting the correct values
and errors when expected.

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And we've seen her do that.