In the previous movie, we introduced the comparison operators with the greater than or equal to operator. Let's quickly take a look at other comparison operators and then we'll talk about logical operators as well. So here are the comparison operators, we have the greater than operator, which allows you to check whether one value is greater than another, the last then we're using the greater than symbol and the less than symbol, we then have the greater than or equal to, and less than or equal to. So those are our comparison operators used very similar to the way we use the greater than or equal to operator in the previous exercise. Now what if you were in a situation where you wanted to compare multiple values, and you didn't want to execute code unless conditionals for these separate statements, all about waited to true. How would you do that?

Well, that's usually done with logical operators. Logical operators route allow you to combine more than one conditional expression. And then the true or false is determined based upon the combined evaluation of those multiple expressions. We have the following logical operators. The logical and which is done using two ampersands. The logical OR which is done using two pipe symbols.

That's the key. That's usually right above the Enter key on your keyboard. And then we also have logical knot which basically reverses the boolean value true becomes false false becomes true. Let's look at some examples of how logical operators could be used. I'm going to jump to sublime and I already have some code Code entered. This is similar code to some we used in the previous movie where we are introducing if statements.

We simply declare a username variable. And then we are checking to see if the username variable is not equal to undefined. If it's not equal to undefined, that means it has a value in it. And so we log out user is and we concatenate the username eltz, meaning it is equal to undefined. Then we simply log out user is not defined. Now what if we were creating a website where we would define the username variable, and then as soon as someone connected, we would put a value into that username variable to track who the user name was.

However, if they did something where they laughed or for some other reason, we wanted to indicate nothing. This again, we would set that username, variable to know. Well, this particular if statement, if this variable is set to no is not quite accurate. For example, let's go ahead and set the username variable equal to null. Now, let's open up the HTML page and see what we are given up the console and see it It indicates the user username variable is defined it has something the problem is what it has is no, which is the value we use to indicate nothingness. So how would we correct that?

We want to check to see whether it's not undefined and whether it's not. No, we want to check both. Both of those need to be checked before we will log out what the user Name is and so to do that we use a logical and that is done you're typing two ampersands. And we enter username. not equal to. No.

Now in order for, for this conditional to evaluate to true, both sides of this logical and must evaluate to true. So the username cannot be equal to undefined and the username cannot be equal to No. If both of those evaluates to true, then the entire conditional is true. And then it will execute this statement. Otherwise, it will execute this. So if one of these is false, it will always execute this.

So the way that conditional and works is if any value evaluates to false, it becomes false. The conditional or the way it would work is that if any value if anything evaluates to true through the entire thing is true. So that's a good way to remember the conditional AND and OR. All right, we made that change where we added the conditional. And let's go ahead and check and see what happens. Now, I'm going to refresh this page.

And notice it now realizes the user is not defined, because it is set to null. And just to test the other side of this, let's set it to an actual value. Save that jump back. And user is James. It shows us correctly. Alright, now how can we do the same statement with the logical OR?

Well if we were to replace this with or, remember if either side of this is true, it will evaluate to true. So for example, let's come out this line, that means it's undefined. So if username is not equal to undefined, that evaluates to false or username is not equal to null that evaluates to true. The whole thing evaluates to true, it will do this, which is not what we want. So in this case, we wanted to use an or we need to set whether these are equal to each other. Set check to see if username is equal to undefined or username is equal to No.

And then if that is the case, we will console dot log that the user is not defined eltz will console log the username. All right, let's see if this works for us with these changes, using logical OR and then making change to how the if statement handles that. So I'm going to save it. Jump out to the browser again. refresh it indicates user is not defined. That's what it should indicate, since we haven't given it a value.

Now let's set that to null. And see if that evaluates correctly. user is not defined Good. Now let's set that to an actual value. Save it and that displays the username. So that is working correctly.

Now the only logical operator we haven't looked at is the not logical not. So let me give you an example of how we might use that. So I'm going to replace this code with some code we used in a previous movie. This had to do with determining whether someone was old enough to vote. We set the can Vote variable to false. And then we check the age.

If the age is greater than or equal to 18, we set can vote to true. If can vote is true console dot log, you are old enough to vote. Well, the NOT operator simply reverses the value of a Boolean. So for example, if we were to put the exclamation point, which is the not logical operator, basically this is going to anything that is true is going to make false anything that is false is going to make true. So we would indicate the opposite message here. Console dot log, you are not old enough to vote.

Now you may be asking yourself why why would you use that? Why would you use logical not? Well, I haven't used it a whole lot, but there are situations where it is valuable to test the opposite. And those are situations where I've used it in code, but it's good To be aware of because you may see it in other people's code or you may run into a situation where it can help you solve a problem. All right, those are the comparison operators. greater than, less than, greater than or equal to less than or equal to and the logical operators logical and logical OR, and logical not.

Let's move on to the next movie.