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In this video we're going to take just a couple of minutes to talk about errors in node j s so we're

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gonna take our working program and we're gonna break it generating an error message.

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Then we'll look at its structure and figure out how we can use the information it provides us to easily

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figure out where in our program things are going wrong.

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And my hope is that this very short video can save you much more time in the long run by making it easier

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for you to find the issues in your application.

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So down below we're going to take our working app and break it by making a change that's going to cause

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an error to show up to do this.

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I'm going to change the code and save notes right here.

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We create a data Jason variable and then we reference it writing its content to a file.

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All I'm going to do is change the upper case as to a lower case one that's going to cause the script

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to crash as we're trying to reference a variable that's never been defined.

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Now let's go ahead and actually run the program to see the error message.

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We're gonna get down below.

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We have to use a command that would actually have save notes get called.

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That could be remove or add.

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I'll just use add.

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So node app dot J.S. add I'm going to add a new note with the following title.

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I'll use something like error message.

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Then I'll set up my body as well.

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And right here I can just use.

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Run it.

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Perfect.

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Now if I run this it is going to have the effect of running save notes when it tries to save our newly

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added note to the data store and the program is indeed going to crash.

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So let's go ahead and hit enter to generate the error message.

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Now if we scroll up we can break this down and the most important piece is right here after a new line

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maybe about four or five lines down.

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This is the actual error message.

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This is the explanation from v 8 as to why your program cut it run.

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In this case it is a reference error which is a whole category of errors and it has an explicit message

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data.

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Jason is not defined so this is letting us know exactly why things failed.

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But it's not really letting us know why.

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So it's still going to be up to us to figure out what's going wrong is data.

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Jason not defined because I've misspelled it or is it not defined because maybe Jason dot string ify

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didn't give us back what we thought it was going to give us back.

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So when we referenced it later it broke.

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So it'll still be up to us to really work through the problem that caused the error but the error can

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get us to the right part of the code.

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Now below the message what we just explored.

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We have a stack trace this contains a trace of all of the functions that are running to get to this

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point in the code and you'll notice the first line right after our message contains the call to save

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notes.

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So it's saying that we are at Save notes and it points to the correct file that is in Notes app.

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It is called Notes Doc J S and it even points to the line and the column so line 56 column 36.

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That is exactly right here where the problem occurred.

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So that first line after it your message contains the most useful information in the stack trace.

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As we go down the trace we typically tend to get less actionable information from here we can see that

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save notes was called by add note which was true that was called by handler which is true handler being

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the method we defined an app dot J S on line at twenty five.

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And if I go over there that's exactly what we're going to see.

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We have our handler method and on line twenty five we do indeed call add note.

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So it save notes to add note to handler and then after that we start to get into the internals in this

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case a lot of these are coming from yards we can see things related to run command pass args pass and

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others and the further we get down the list the closer we get to the node internals which in this case

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are these last three.

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So when we're looking at the stack trace we want to start from top to bottom as the top contains the

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most explicit pertinent information related to the error.

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That's where I'd like to stop for this video.

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You now know a bit about how to work with error messages and how to use the no debugger to debug your

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node applications using the Chrome developer tools.

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These are both techniques you can now use throughout the rest of the class to fix your issues faster.

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Now you're still going to make issues that is expected and completely normal.

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But it does get better over time as you get more comfortable with node you'll make less and less issues

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for yourself to solve and the ones you do create you'll be able to solve faster.

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But like everything it is a skill set that needs to be built up over time.

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We're gonna start to talk about asynchronous development in the next section.

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So let's go ahead and jump right into that.