What does the % symbol do?

- Hey, look what I found, a calculator. Know how on the calculator you have this little percentage symbol right here? So if you go to a restaurant and get a bill of $19 and you go I want to pay 15% tip on this bill. Then you would go 19 times 15 and then hit the percentage button and you get the percentage, so that's 2.85, so that would be 15% of 19 is 2.85. That's how this percentage symbol works on a calculator. What if you passed this here to JavaScript, what do you think happens? So 11 % or percent symbol, three. The answer to this in JavaScript is two. That makes sense, right? Well, you see, in JavaScript this is not a percent symbol, this is a symbol for modulus or remainder. So what is actually happening here is we're saying if you take 11 and then you use the number three and you see how many threes you get here and then I'm only interested in what's remaining. So basically we say 11 is a one, two, three, four, five, six, seven, eight, nine, 10, 11. And then we say how many threes can we make here. So that's three here, and then another three, and another three, and the remainder are these two. So that's this number you get here. Which begs the question, why on Earth do this? Well it has some practical abilities that can be used for things. For example, let's say you want to figure out if a number is a prime, and this is something you need for I don't know, you might need it for some various reasons. So if you don't know what a prime number is, a prime number is a number that can only be made up of the number one, so just a bunch of ones, or itself, or to put it another way, it's a number that can only be divided by itself or the number one. So if you use this modulus function in JavaScript, you can actually figure out whether or not any number is a prime number. And the way you do that is you say, if you take the number and then modulus any other number that's lower than that number, if you get anything other than the number zero, it means that you're likely looking at a prime number. But if for any of those calculations you get the number zero that means that you can actually divide the number by another number, then it's not a prime number. Case in point, is five a prime number? Well, if we do five modulus one, we get zero because there is no remainder. If we do five modulus two we get one because we get two twos, so two plus two is four, and then you have one left. If you do five modulus three you get two because you get one three in five and then you have two remainder. If you get five modulus four, you get one because you have a four plus a one. And if you get five modulus five you get zero because whatever. But, if you do a number that is not a prime number, like six for example, then you say six modulus one is zero because there is, you don't get any remainders here. But six modulus two is also zero because you can put three twos in a six. And then you have no remainder. So already in this test here we already know this is not a prime number. So to find prime numbers using the modulus in JavaScript, what you do is you take the number and then you increment up from one and you say any number that's higher than one and lower than the number you're testing, if the modulus at any point is the number zero, then that's not a prime number. That's one way of using modulus. Of course there are other things you can do with it too, just don't think you can calculate percentages because that's not what its for. I don't know why that is, but for some reason they decided to use this symbol for something completely different than what it looks like.