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For most of JavaScript's life there's
been only one way to create or

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declare a variable, the var keyword.

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Although many JavaScript programmers
still use the var keyword,

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some of the behaviors of using var
can be problematic or unexpected.

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For instance, a variable might
accidentally get changed or

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overwritten with a different value
further down in your program,

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which can be really difficult to catch.

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The JavaScript programming
language gets updated regularly.

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And nowadays you'll see variables
written using the keywords let or

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const, which let you put a value into
them like you do with the var keyword.

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But they have different rules
about how you use them and

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change their values in your program.

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So now I'll teach you how const and
let work and

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demonstrate why you might want to use them
instead of var when declaring variables.

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>> Let and const work similarly to var.

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In each case you use the keyword var,
const, or let followed by a name for

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the variable, an equal sign, and
a value you want to put into the variable.

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We started with the var keyword since it's
a little easier to remember that var means

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that something is a variable.

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And you'll still see var keyword used to
declare variables, but modern JavaScript

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development has shifted over to let and
const because of their benefits.

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In many cases,
they can make your code less error prone.

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So what's the difference between var,
let, and const?

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And when should you use
one method over another?

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Well we're not far enough into
JavaScript to truly understand and

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benefit from the usefulness of let and
const.

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But I'll teach you the basics of each
starting with the const keyword.

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In the JS folder,
open the file named let-const.js.

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We'll now work in this file, so let's
make sure to update the script tag and

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index.html so
that it points to the let-const.js.

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You don't need to write your
variables in separate file like this,

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it's just a new file we're using to
learn about the let and const keywords.

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So const is often considered the best
choice for declaring variables.

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Const is short for constant, meaning that
the value of the variable should not

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change, it should remain constant.

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Const lets you protect the variable from
being overwritten by any accidental or

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stray assignments in your code.

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In other words,
once you create a constant and

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assign it a value,
you can't assign it another value.

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Let me show you what I mean.

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Let's again declare a variable,
or in this case a constant,

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called score, and assign it a value of 5.

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Then use console.log to display
the value of score in the console.

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When I refresh the browser, I see 5
logged to the console, as expected.

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Earlier, you learned how to use
the addition assignment operator

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to manipulate or add to the content stored
in a variable, like a score for example.

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So let's try to add 5 to the score
constant with score += 5.

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This time we get different results.

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There is an Uncaught TypeError
in the console

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that says assignment to constant variable.

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If there's one detail you should
remember about using const is that

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you cannot change or manipulate the value
of a constant through reassignment.

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You're also not able to
re-declare a constant.

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For example, re-declaring score and
setting it to 5 produces

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the Uncaught SyntaxError identifier
score has already been declared.

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The same happens if I re-declare
using the var keyword.

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You'll use const variables frequently to
store values that shouldn't change while

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the program runs, like product prices,
a password, or username, for example.

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You can assign any value to a constant.

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In a later course you'll, for instance,

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use constants to select
elements of a web page.

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Imagine an application with hundreds or

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even thousands of lines of code that uses
many variables that should not change.

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If you're declaring those
variables with var,

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it could cause bugs that
can be hard to find.

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For example, a score, product price,

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or user name might unexpectedly
get overwritten entirely.

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So the const keyword is one safeguard
to prevent that from happening.

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For this reason, const should be your
first choice when declaring variables.

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However, for a variable whose value
should change while the program runs,

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constants are not the best choice.

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For example, you wouldn't const
to store the score of a game

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since you know that the score needs to
change as the player plays the game.

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In that case, use the let key word
when you want to reassign a variable.

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In that respect, let works just like var.

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So let's declare let score

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= 5 and score += 10.

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The console outputs the expected value,
15.

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The let keyword is almost the same as var,
it lets you define a variable but

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limits how you can use it.

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For example,
let's say that later in your program you

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re-declare a let variable,
like let score = 20.

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This produces an Uncaught SyntaxError that

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says identifier score has
already been declared,

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whereas var would let you overwrite
the value in the initial assignment.

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For example, if I change each let to var,

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Notice how the value of score is now 20.

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This behavior could produce
bugs in your program

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because you might accidentally
use the same variable twice.

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So let offers more protection and
helps prevent you from doing that.

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There are other details and
differences between let and

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const which you're not
required to know about yet but

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you'll learn more about them in
later courses and workshops.

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Let and const will come in handy when
you start working with functions, for

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example, and when you learn
something called scope, the area or

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context in which variables
are accessible inside of your program.

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Since let and const are considered
safer choices for declaring variables,

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I'm going to use them moving
forward in place of var.