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Now for the most part working in numbers and Javascript is pretty straightforward.
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So for example you can add by simply using the plus sign and you can subtract by simply using the dash
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or the minus sign. When you want to multiply,
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you can use the asterisk as the multiply sign, and division is done with a single forward slash.
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Now the only one operator that's a little bit weird is the modulo, and it's represented by a percentage
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sign.
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It looks almost like we're trying to divide nine by six,
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if you just try to imagine the similarity with the division symbol, but it's actually going to give you
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the remainder of the division.
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So 9 divide by 6.
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So 6 goes into 9
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only one time, and you get a remainder of 3, which is going to be the outcome of this expression.
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So we know that 6 divided by 4 is 1.5, but 6 modulo 4 is going to give us 2, because that's the remainder
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of what 6 divided by 4 will be.
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Now what do you think 12 modulo 8 will give you?
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Right.
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So 8 goes into 12 only once and the remainder is 4.
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Now why is this useful?
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Why do we even have this operator?
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Well, if you are trying to work out if a number that you have is odd or even, say 45, you could divide
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it by 2,
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and if the modulo is 0 then it will be a even number,
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but if the modulo is not 0 then it will be an odd number.  And there's various other ways that you can
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use the modulo in your programming, which we'll come to see in our future lessons.
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Now that was pretty easy, right?
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But what about this expression?
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Say if we create a variable called cost, and it's equal to 3 + 5 * 2.
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What does this equal? If we first added 3 to 5,
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then we multiply that by 2,
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we would get 16.
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But if on the other hand we first multiply the 5 by 2 and then add 3, we get 13.
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Now in math the ambiguity is resolved by something called precedence.
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There's a rule that says we must first multiply and divide before we add or subtract.
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And this works exactly the same way in programming.
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So this expression will evaluate to 13 because it will carry out the multiplication first.
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Now if we wanted to get 16 from this expression then we can use a set of parentheses to make our intentions
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clear, to say that we first want to add 5 to 3 before we carry out the multiplication.
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And in order for your code to be easily readable and easily understood, it's good programming practice
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to add a set of parentheses even when you know that the multiplication is going to happen first.
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This makes it abundantly clear what is going to happen first and what is going to be the outcome of
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this expression.
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All right. So now it's time to use what you've learned and complete a challenge.
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I want you to create a dog age to human age converter.
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So how old would your dog be if it was a human?
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And this is the formula that you're going to use. And this formula is completely valid from a mathematical
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perspective.
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So you're going to subtract 2 from the dog age first, then multiply 4,
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then add 21. And I want you to use what you've learned to create this converter by going into the Chrome
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Developer Tools, going into Sources and creating your code inside your index.js snippet.
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So your code is going to create a prompt to ask the user for the age of that dog.
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Then it's going to calculate the equivalent human age of the dog using this formula.
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And then it's going to give this answer back to the user through an alert.
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So pause the video and see if you can complete this challenge.
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All right. So let's tackle this.
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Now the first thing we're going to do is we're going to create a variable called dogAge, and this is
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going to be set to equal the answer that we get from the prompt where we ask the user 'How old is your
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dog?'.
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Now once we've got that number then we can carry out the formula.
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So our formula states that in order to calculate the human age we have to subtract 2 from the dog
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age, and then we have to multiply this by 4 and then we add 21.
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Now this, as I've written it here, is a very ambiguous statement.
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I have no idea which part is going to be carried out first and which part last.
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And this will likely not give us the right answer.
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So let's go ahead and add some precedence rules and also make our expression more clear by adding in
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some parentheses.
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So the first thing that we need to happen is that dog age needs to be subtracted by 2, and then it's
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going to be multiplied by 4,
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and finally it's going to have 21 added to it.
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Now, even though the second set of parentheses were not entirely necessary,
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it does however make our code look abundantly clear as to exactly what the order of the calculations
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are going to take place.
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So finally we can send an alert back to our human user and we can say, your dog is this many years old
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in human years.
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All right. Cool.
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So let's hit run and give this a go.
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How old is your dog?
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My dog is 7 years old.
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Let's hit
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OK.
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And it tells us that the dog is 41 years old in human years.
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So that was pretty cool.
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Did you get it right?
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If not, take a look back at this lesson and try to refresh yourself on some of those concepts.
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So in the next lesson we're going to look at some slightly unfamiliar mathematical operations that you
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can do with Javascript numbers that you might not have seen in maths.
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So for all of that and more, I'll see you on the next lesson.