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Every condition evaluates to either true or false. True and false have a specific meaning in JavaScript, they're called "Booleans," and along with strings and numbers, they represent a third type of value (or data type) in JavaScript.
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A condition in a conditional statement
is really just a question with a yes or
0:00
no answer.
0:04
In JavaScript, we say that the condition
evaluates to either true or false.
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True and False have a specific meaning
in JavaScript, they're called Booleans.
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And along with strings and
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numbers, they represent a third type
of value or data type in JavaScript.
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Though they have an unusual name,
Booleans are simple.
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Remember, a Boolean value can be
only one of two possible values,
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true or false, that's it.
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There will be times where you'll want
to ask your program yes/no questions,
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then run different code
based on those answers.
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Let's have a look at some examples.
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This time open the booleans.js
inside your js folder and
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update the script tag in index.html so
that it points to booleans.js.
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In JavaScript programming, the two
possible Boolean values are represented by
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the keywords, true and false.
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Because all conditions return a true or
false value,
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you can see how these two keywords work
by using them in this if/else statement.
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If I type true inside the condition and
refresh the browser,
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The condition is true
displays in the console.
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Now, if I type false inside the condition,
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The condition is false appears.
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Notice that there are no quotation
marks around the words true or false.
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They're not strings, they each represent
a specific type of value, a Boolean value,
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so quotes should not go around them.
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You can assign a Boolean
value to a variable,
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just like you can assign a string or
a number to a variable.
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This is a common way to keep track of
a certain condition throughout a program.
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For example, let's see how we can use
a Boolean value in a number guessing game.
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First, declare a new variable
using the let keyword named
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correctGuess, and
set it to the Boolean value false.
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When the number guessing game begins,
the player hasn't yet guessed anything, so
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we'll say that the guess is false.
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The value of this variable will only
change if the player gets the correct
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answer.
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And notice how we're using the let keyword
here because we expect the value of
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correctGuess to change.
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Next, declare a variable named number
to store the number two guess.
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You can set it to any number between 1 and
10, I'll set it to 6.
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Then let's declare one
more variable named guess.
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To store the answer,
the user types into a prompt dialogue,
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which will instruct the user to
guess a number between 1 and 10.
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Next, below the guess variable,
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let's write a conditional statement
that changes the correct guess
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variable from false to true if
the user guessed the correct number.
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Type, if guess === number,
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run the code inside these curly braces or
code block.
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If the number the user typed
into the prompt dialog exactly
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matches the number stored
in the number variable,
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we're going to change the value of the
correctGuess variable from false to true.
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If the player guesses wrong, the variable
isn't changed, it remains false.
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Now, there's one detail we need
to address in the condition.
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You learned that the value returned
by the prompt method is a string.
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So notice how in the condition,
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we're comparing a number
value with a string value.
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For example, even if the user types 6,
the program will compare
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the number 6 inside quotes with
the actual number value 6.
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And this is always going to return false,
because we're using the strict equality
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operator, and
the game will not work as expected.
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So for the sake of this example,
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we need to convert the string value
returned by prompt to a number value.
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The quickest way to do this
is by adding a plus symbol or
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the unary plus operator just before
the guess variable like this,
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make sure that there is no space
between the plus and the variable.
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So now, this ensures that we're
comparing two number values.
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It may seem a little strange, but
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don't worry too much about
this operator for now.
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You're going to learn a whole lot more
about converting strings to numbers and
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working with number
values in a later course.
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For now, I've posted more information
about the unary plus operator and
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the teacher's notes with this video.
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Finally, let's update the conditional
statements below to check the current
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value of the correctGuess variable and
put a message that matches the condition.
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First, the if statement will check if
the value of correctGuess is true.
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If it's true, then log the message,
You guessed the number.
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Else, if the value is false,
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then print a different message
that reveals the variable number.
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I'll use a template literal
to insert the value
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of the number variable Into the message,
Sorry,
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the number was ${number} and that's it.
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Before running it,
let's step through this code line by line.
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First, we declare a variable
with the Boolean value false.
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Next, we store the number to
guess in a variable named number.
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We then open a dialog box and
ask the user to supply a number,
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which we store in the variable guess.
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Then the program tests
the user's response.
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If the user supplied the same
number stored in guess,
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then the correctGuess
variable changes to true.
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If the player didn't however, then
the variable stays the same, it's false.
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Then we have a second
conditional statement that says,
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if the value in correctGuess is true, then
print a winning message to the console.
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However, if the value is false,
then reveal the number.
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All right, let's see it in action.
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I'll save the file, refresh the browser.
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If I type, say, 8 into the prompt dialog,
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the console displays,
Sorry, the number was 6.
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Refresh the page to try again,
this time I'll type 6 and
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see the message,
You guessed the number, good.
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You can also simplify the code
in this if/else statement.
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Remember that all conditions
are either true or false,
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that is, the test condition
produces a Boolean value.
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Since we're already storing either true or
false in the correctGuess variable,
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we don't need to use the strict
equality operator at all.
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In other words, this is the same as this.
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And as you can see,
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the number guessing game still
works in the same way as before.
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There are other ways you might
write this number guessing game,
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without having to use two
separate conditional statements.
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I use this approach to teach you
how Booleans get evaluated and
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change the flow of a program when
used in conditional statements.
7:28
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