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Introducing the Practice
4:02See the program you'll create in this practice.
[MUSIC]
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Hey everyone, Dave here.
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Let's practice some
JavaScript programming.
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This practice session covers
basic conditional statements.
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It's a great follow up to the fourth
part of the JavaScript basics course.
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If you haven't taken that yet and
find this practice session too difficult,
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then go through the first
four parts of that course.
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I've added a link in the teacher's notes.
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For this practice session,
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you'll start with an already
functioning JavaScript program.
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In fact, this program was created
in an earlier practice session.
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If you haven't done that one yet,
you can go through it first, and
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then come back to this one.
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See the teacher's notes below for
a link to that workshop.
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Let's look at how
the program currently works.
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It accepts two numbers as input,
then does some simple math and
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writes the results to the page.
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But there are some things a user
could type in that would cause
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this program to not work so well.
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For example,
division by 0 is not allowed in math, but
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a user could type 0 for the second number.
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Or someone could type a word
instead of a number, or
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even leave the prompt field empty.
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Wow, now that really doesn't work.
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NaN stands for not a number, and
it's the value you'll get when you try to
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perform math using letters or
try to evaluate a string as a number.
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So the program works some of the time but
not all of the time.
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In this practice session,
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I want you to add a conditional statement
to control the flow of the program.
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And to deal with the two
situations I just showed you.
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Here's how the completed
program should work.
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If I type 0 for the second number, I
should get an alert dialog that says that
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division by 0 isn't allowed,
and I should try again.
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I'll reload the page and type
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hello in the first prompt, and
leave the second prompt empty.
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Now I'm told that one or
more of the entries aren't a number, and
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I should try again.
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Great, and
it still works if I type numbers.
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Go ahead and
launch the workspace on this page.
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There's a file named math.js and
one called instructions.txt.
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The instructions file contains
the requirements for the project.
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The first instruction asks you to
move your variable declarations to
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the top of the file.
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Now this isn't a strict
technical requirement for
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the script to run,
it's more a professional best practice.
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Programmers recommend moving variable
declarations to the top level of the code.
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This way, you can immediately see
the variables used in the script.
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You can declare them even
without adding a value to them.
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Let me show you one example now and
you can do the rest.
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At the top of this file,
I'll declare the first variable.
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I could put the prompt here too,
but I won't.
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I'll leave that for later in the program.
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Now I'll also remove var from here.
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You only use var once, when you first
create or declare the variable,
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not when you change the value in it.
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Okay, you can move the rest of
the variable declarations yourself.
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Read the instructions.txt file for
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the requirements on how
the conditional statement should work.
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One thing you'll need to use
is the is not a number method.
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This method checks to see if
a variable contains a value.
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It looks like this.
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You put the name of the variable you
want to test inside the parentheses.
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It's a JavaScript function
that returns another value.
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In this case,
it returns a boolean true or false value.
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If the value in the variable is not
a number, it returns the value true.
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Remember, boolean values are the key
to conditional statements.
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I provided a couple of links below to
explain the is not a number method and
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to remind you of what and
how boolean values work.
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If you want to do some quick studying
before tackling this practice session,
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in the teacher's notes, look for
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links to videos that can help
you solve this challenge.
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But if you're ready, go for it.
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Program your solution, and
I'll show you mine in the next video.
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