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The solution for the practicing let and const variables in JavaScript.
Using Node in Workspaces
The instructor uses the Workspace console and the node command to run and test their JavaScript file. Learn what that means.
More about Let and Const
Define Variables with let and const
More about Objects
Intro to Template Literals
String Interpolation with Template Literals
Spread Operator
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Hi, again.
0:00
How did it go?
0:01
Did you have any trouble?
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If you did, that's totally fine.
0:04
Keep watching to see my solution, and
then go ahead and give it another go.
0:06
If you open up the let_const.js
file in workspaces, you'll see
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two variable definitions, teachers and
courses, toward the top of the file.
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These two variables were
declared using the const keyword.
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This means that the variables
can not be reassigned.
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Your first task in this practice
session was to write a function that
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updated this teacher's array.
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To do this, you may have tried to
reassign the teacher's variable
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by setting it equal to a new array
that included all of the teachers.
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If you try this,
you would have received an error message,
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because const variables
can't be reassigned.
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But just because an array was
declared using a const variable,
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it doesn't mean that the array
itself can't be changed.
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You could update or add to our teachers
array by using the push method and
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the spread operator like I did here.
1:01
Don't worry if you haven't seen
the spread operator before.
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It's a handy new syntax in ES2015
that lets you use iterables,
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like arrays, in places where you'd
normally expect multiple elements.
1:13
For more information on this,
check out the teacher's notes.
1:18
Your second task was to fix
the printTreehouseSummary function so
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that it outputs the correct
number of courses and teachers.
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You'll notice that at the top of
the file we declared a variable i
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using the var keyword, and set it equal
to the length of the course's array.
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But uh-oh,
in our printTreehouseSummary function,
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we're using that same
variable i in our for loop.
1:42
When our for loop is initialized,
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our existing variable i becomes shadowed
by the i variable we declare in the loop.
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This means that inside
our loop our existing or
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global variable i is being overridden.
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So before the end of our function, instead
of being equal to the number of courses,
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i is actually equal to
the number of teachers.
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Whoops!
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This is a pretty simple fix though.
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Thanks to the addition of
let variables in ES2015,
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we can have another variable
i that is block scoped,
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meaning it's only accessible in
the code block where it's used.
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In our case, that's our for loop.
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By changing var to let in our for
loop, we've eliminated our problem.
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I should also note that i is not
really a meaningful variable name, and
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problems like this can be
avoided by making sure that
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all of your variables have clear and
meaningful names.
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Now, when you try to run your code at
this point, you got that error message
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I mentioned in the first video, were
you able to figured out what caused it?
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If you guessed that the error was caused
by passing the newTeachers variable to
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the addNewTeachers function before
it was declared, you'd be right.
3:02
Variables declared with the keyword var
can be accessed before they are declared.
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let and
const variables differ in this way, and
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you should always declare them
at the top of their scope.
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By moving our newTeachers variable
to the top of our file or
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simply before we call the addNewTeachers
function, Our code will now run.
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Let's try it out.
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Awesome, our code now runs and
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prints out the proper number of
JavaScript courses and teachers.
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If your code isn't working or
your output doesn't look like mine,
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go back through this video and
check your code against my solution.
3:52
Also, don't hesitate to reach out to other
students in the Treehouse community.
3:56
Talking about your programming challenges
with other developers is another
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great way to learn.
4:04
Hopefully you had fun with
this practice session and
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feel more comfortable using const and
let variables in JavaScript.
4:07
Talk to you soon, and keep on coding.
4:12
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