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Hi there.

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In this lesson, we are going to talk about error handling and to try and catch statement when we are

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developing software or web applications, we need to make sure that we handle the errors, avoiding

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our program from breaking.

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So let's get started with an example from the previous lesson before we had that callback function,

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when we added the time out and then we tried to use this user in the great user function.

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Let's remember that we got an error.

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So I'm just going to save this and refresh the page.

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And now, as you can see, we're getting an error.

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And when we get an error like this, let's see what happens when I write more code below it.

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So I'm just going to write a message here.

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Just going to write a new line of code, let's save this.

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Let's go back there.

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I'm going to refresh the page.

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And as you can see.

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Nothing that we write after the error is executed, and this means that when this happens, all the

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functionality of our page can be compromised.

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So how do we tackle this?

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How do we protect our application from breaking when we have errors?

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For that, we are going to use the try and catch statements.

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So going back there, instead of just trying to use this information, I'm going to start a try block.

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Like this.

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And I'm going to place this inside the tri block, so here we were simulating as if we were getting

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this information from an external source.

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So we are not sure if we are going to get this information right.

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We are not sure what's going to happen because this information is coming from outside in this simulation.

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So when we work with things like this, we should always use a try.

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So here we are going to try to do this.

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And after the try, we're going to add a catch block.

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And here we are going to pass an argument for the error.

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I'm going to talk about it in a moment.

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And inside here.

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Let's write another message, so this time let's just write high guest user.

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How are you?

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So what's going on here?

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I'm trying to get this information and sending this message, using that information.

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So the way this works is.

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We're going to go inside the try block, and if we can do everything without any error, we are just

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going to stop here.

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So if we can get the user and then call this function and there's no error, fine.

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We are just going to stop here.

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However, if there is an error for some reason, instead of breaking our program, we are going to jump

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directly into the catch block.

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So if an error occurs, it means that we were not able to get the user.

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So let's not send the message using that information.

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Let's just send a.

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Generic message.

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So now let's save this and let's see the difference.

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I'm going to refresh the page, so now there was an error with the message that we wanted to send,

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but instead of breaking the code, we just jumped to the catch block.

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Now we were able to send a generic message and everything.

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We right after that is just going to be executed normally, which is exactly how it was supposed to

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work.

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So now we are protecting our application from breaking.

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Now let's go back to Visual Studio Code and talk about this argument that we passed.

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This is actually the error message because using try and catch statements is really good to avoid the

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application from breaking.

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But if we are debugging it, if we want to know why we are getting an error.

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It's really hard because we don't have anything in the console, so bypassing this argument, let's

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do a console that log.

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And pass this this name can be any name you want, but this is a convention, so I suggest you always

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use this because every programmer that sees this will know that this is the error message.

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So if we do this, let's save this.

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Refresh the page.

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Now, as you can see, we are able to see what happened still our application did not break, but we

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were able to get that message.

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And if we were debugging this to understand.

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Why we have an error, it would be way, way easier to fix this issue.

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Now let's talk about the finally block.

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So going back to Visual Studio Code, besides using try and catch, we can also use a finally block.

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So this is optional.

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You don't need to use it if you don't want, the only statements that are mandatory are to try and catch

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statements you cannot use to try without the catch.

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You always need to use both, but you also have the finally, which is optional.

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So let's try to put this message inside a finally block.

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Let's right inside.

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Finally, block, so let's save this, refresh the page, actually, let me go back there just to comment

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this line so we don't get distracted by this.

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All right, refreshing the page.

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As you can see, this is a generic message, which means we had an error.

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We went to the catch block, but now that line of code is executed anyway.

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And now let's try removing the error, so this time we are not going to have a set time out.

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Which means that we won't have that error, so let's refresh the page and now, as you can see, this

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is not a generic message anymore, which means we didn't have an error.

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But we also have this line of code being executed.

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So this is how we use the final block, this is a block of code that you can put here to be executed

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no matter if you have an error or not.

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I actually don't see the point of using the final block, because if you think about it, if you leave

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it outside, it's just the same thing of using the finally because this is going to be executed anyway.

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So I think this is more a matter of organizing your code if the things you are doing inside here are

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related.

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So this is how you use the final block.

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And as I said, this is optional.

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So if you don't want to use it, you don't have to.

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Now, before we finish with this subject, let's talk about the throe statement.

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So for this, I'm going to go up here and let's leave it like this.

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So we won't have any error, but now let's let's leave it blank, so let's use an empty string for the

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name.

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I'm going to save this.

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And how refreshing the page, actually.

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I'm just going to comment this line as well.

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Let's just leave the try and catch blocks, so let's refresh the page and see what happens.

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Now, as you can see, there were no errors because after all, we had this information.

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We had a variable with an object.

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This object had a name property.

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So there were no errors.

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But now we have a weird message because even though we received a user, the name was blank.

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So it would be useful to create our own customized errors.

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And this is what we can do with the throe statement.

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So now let's go back here.

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So first, let's get the user and now we know that any JavaScript error is going to make this go directly

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to the catch block, but we can also create our own errors.

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So if the username is empty, which we could do it like this.

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So if user not name.

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Is the same as an empty string, we could write it like this or we could just use the exclamation mark,

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which is the same thing if there's no username, which means that if it's zero or if it's an empty string,

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we are going to consider this as an error.

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So we are going to throw an error.

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And we can right any message we want, so in this case, we're going to write name is empty.

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So what's going to happen here inside the tri statement, besides looking for any error, every time

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we hit a throw statement, we are also going to be sent to the catch block.

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So when the interpreter hits this, it's just going to stop everything inside here and go directly to

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the catch block.

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So now let's try again.

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Refreshing the page, as you can see, we still have no error here, but instead of just using that

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empty string, we consider that as being an error.

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And now we are going directly to the catch block, which is really good.

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And why did we write this?

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Well, this is the error message.

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So if you are debugging your application and you want to understand what happened here, why are you

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not sending a message?

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If we send this error to the console.

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You can see that the string that we pass in the troll statement is going to be the error message in

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case that happens.

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So in this case, the name was empty.

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So we just created an error.

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And this is the error message which makes this go directly to the catch block.

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And the message we wrote is going to be passed as the error message.

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So this is how we use try and catch blocks to avoid our application from break.

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So that was it for now.

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I'll see you in the next video.