- Welcome to Getting Started With ES2015 1:01
- Declaring Variables in JavaScript 1:20
- Creating Unchanging Variables With const 4:10
- Using Constants with Arrays and Objects 2:44
- Review creating variables with const 5 questions
- Defining a Variable with let 2:48
- Using let with for Loops 2:25
- Reviewing let and const 1:00
- Review let and const 5 questions
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Creating Unchanging Variables With const
4:10Using the keyword `const` to create variables have a number of benefits including the ability to prevent the value from being reassigned.
The first method you should consider when
creating a variable is the const keyword.
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This is often the best choice for
declaring variables.
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Const is short for constant, meaning
that the value of the variable shouldn't
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change, it should remain constant.
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For example, const pi is equal to 3.14159.
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Const lets you protect variables from
being overwritten by any stray assignments
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in your code.
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In other words, once you create a
constant, you can't give it another value.
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Let me show you what I mean.
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If you want to follow along, you can
open up the workspace on this page.
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Open up pi.html.
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And as you can see, we've assigned
the value of 3.1459 to the constant pi.
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Click on preview and navigate to pi.html.
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Open up the developer tools and
head over to the job script console.
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When we type pi, The value is locked.
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Try to assign another value to pi.
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We see we get an error.
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TypeError: Assignment
to Constant Variable.
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You will use constants frequently,
in fact,
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it should be your first choice
when declaring variables.
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You will use them to store numbers that
shouldn't change when your program runs.
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For example, tax rates, dimensions
of user interface components and
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product prices are good examples.
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And you'll use constants when you're
selecting elements on a page.
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Or when you're assigning
a function to a variable.
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Any value can be assigned to a constant.
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However, for a variable whose value will
change during the life of a program,
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const are a bad choice.
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For example, you wouldn't want to
const to store score of a game.
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Since the score changes as
the player plays the game.
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In that case, you'll use the let keyword.
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I'll show you how that
works in another video.
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As I mentioned,
constants help you fix problems
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that can easily crop up in
your JavaScript programs.
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Let's take a look at a common
problem that you'll run into.
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If you want to follow along, open up the
name.html file in this video's workspace.
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Here's a simple script
with a variable name.
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I'll preview this file and
check in the console.
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Notice that the console says Andrew.
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That's the value of the name
variable defined on line one.
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The console then prints out Joel Kraft
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which is logged by
the createFullName function.
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Now that the program has run,
what's the value of the name variable?
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I will just check that out in
the console by writing name.
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It's now Joel Kraft.
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Andrew is gone.
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The variable has been
overwritten by the function.
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This is a simple scope issue.
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The function is overwriting the name
variable in the global scope.
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It's a problem that can pop up and
can be difficult to catch.
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However, const will help
you spot the mistake.
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Now when I preview this,
The console spits out an error.
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You can't change the value of a constant.
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Because I use const, I can see that
the simple mistake I made, and
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I can go back into my script and fix it
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Simply adding const before
the name within the function,
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keeps that variable locked to
the scope of the function.
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It's no longer trying to
reassign the name variable.
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I'll preview this script one more time.
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And no errors, and I can logout name,
and it's still untrue, hurray!
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I'm not gone.
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