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You guys in this video I want to discuss error handling and Javascript and for the most part what a

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try catch block is.

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So a try block lets us do something let's just write some bit of code and test for errors and the catch

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block lets us handle those errors if there are any.

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OK we can also throw our own errors.

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So let's go ahead and dive into this and let's go ahead and say try.

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And then after that we'll say catch

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and then this is going to take an error object.

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OK.

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Sometimes you just see sometimes just c e r r error.

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I like to use e in this case.

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And then let's just put something in here that will fail.

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All right.

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So let's say my function.

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Right so we're calling My function Now why will this fail because there is no declared to my function.

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It's actually going to be a reference error.

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OK there's different types of errors in javascript this will produce a

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reference error such search capital so reference are all right and then down here we can decide what

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we want to do when this fails.

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So what I'm going to do is just console dot log.

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And when a console log.

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E see what that gives us.

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All right so very simple code.

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Let's go ahead and save this and run it and we get a reference error down here that says My function

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is not defined.

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All right.

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Now what's really nice about try catch is that we can handle errors without stopping the entire script.

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So down here I'm going to say console log and I'm just going to say program continues OK and save and

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you can see that you know we get the error.

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We catch it.

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We do we do something with it which in our case is just console logging.

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And then we continue.

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Now if I were to take this my function and just go like that and save we just get this nasty error down

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here and read and the script doesn't continue we don't get our console log.

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All right so try catches a very elegant way of handling errors.

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Now if you want to just get the message my function is not defined and I get this to the name reference

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error.

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You could do just go ahead and copy this down.

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You could do the dot message and comment this so and save.

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And now you see we just get my function is not defined.

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So if you didn't want to just log this you could if for some reason if you wanted to output it to the

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browser into the dawn you could do that.

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You can do whatever you want with it.

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All right.

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Now if you wanted to just yet type or actually it's called the name which is the type of error you could

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do it name it and save and you see that it'll give you reference error.

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OK you can also test to see what kind of error is.

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So if we were to say console log and inside here will say e and we can use instance of code can use

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the instance of operator and say no reference or

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kind will save and you can see we get true.

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Now if we were to test and see if it was a type error which is a different kind of error we get false.

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All right so you can test to see that but I'm just going to go ahead and comment those out and leave

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just the

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now there is a third piece you could put onto this block and that's finally and all this does.

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Very simple it just runs no matter what.

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No matter what the result of anything up here is this will still run.

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So we'll say console log and we'll just say finally runs regardless of result and save and you can see

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that that's going to run.

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No matter what happens up here.

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Now if we wanted to produce a different type of error.

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So it's produce a type error.

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So what it what we could do here is take now and we could try to call something from it like my function.

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All right we can't call something from now.

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This is going to cause a type error.

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So let's save it and run it and you can see this cannot read our type error cannot read properly my

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function of no.

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OK.

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And you can make this you know this part.

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This right here whatever you want.

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You don't even have to use IE.

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You could say

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console log and you could say

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it's Nelse stupid and that's what that's what it'll show you down here.

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All right so you can format these errors however you want and if you wanted to go back there you could

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actually put in some tactics here and you could put a dot name like that and save and get type error.

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It's null stupid.

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So you can format these however the hell you want.

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All right all right.

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So let's see what other types of errors can we do here.

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Let's do a syntax error right.

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So we'll say it will produce syntax error.

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And what I'm going to do here is I'm going to use the Val function which isn't used too much but this

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will evaluate javascript within a string.

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So for instance we could say two plus two we'd actually have the console log it for it to work or for

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it to show us anything.

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So you can see we get four so we can evaluate javascript.

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Now if we were to do Val and say hello which is going to be look these are going to be looked at as

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variables.

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If we were to put in here quotes like that the more revaluing evaluating it as a string.

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And that doesn't throw an error but if we were to get rid of these double quotes and save that you can

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see that there was a syntax error unexpected identifier.

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So it's talking about this hello it's looking at it as a variable.

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All right.

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So that there was a syntax error.

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That's another type of error we have is you are I error.

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C you are.

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I error.

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And we could use a function called de-code.

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You are a component and then passing something that doesn't make sense.

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It's going to pass in a percent sign and save.

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And we're going to get an error that says you are by error you are malformed at decoded and component.

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All right.

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So just just trying to show you the different types of errors that can be thrown.

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All right so now that you've seen basically just global errors or core errors being thrown I want to

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show you that we can create our own.

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So let's say up here we have a variable called user

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and let's say there is email say Jadot at gmail dot com.

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So let's say we want to check to make sure that this user has a name which it doesn't it just has an

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e-mail.

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So what we could do is go down here and we could say if

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we'll say if no user name then let's throw an error.

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So all we have to do is use the throw key word and then we'll say user has no name.

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So let's save that and we get down here a user has no name and Cain you could put that inside the dorm

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if you wanted to as well and some kind of an alert.

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Now another thing you could do this is just throwing the string but you could actually format it as

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a certain type of error like a syntax error.

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So to do that we could say throw new syntax error and say user has no name and save.

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And now you can see we get syntax error.

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User has no name.

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And it also gives us the line number the file name and line number.

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And again you could format this how you want.

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You could do something like.

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You could say Konsole log

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and you can make up your own type.

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You could say like user error and then just put in here the dot message and save and we get user error.

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User has no name.

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All right so gives you a nice elegant way to handle errors and Javascript also shows you the different

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types of errors that there are.

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All right so that's it.

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And I will see you in the next lecture.