lesson-9 testing questions

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We are writing the code in lottery.once block. if it does not get pass or has an error it shall fail. but here we are using try and catch i see how it functionality works. but why would we want to use it here and even though if something is wrong with the code we will get an error. so what makes try and catch useful in this case?

What's a listener? and what's the use of it?

And How did we get once keyword?

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@krakxn You think you could help me with answers? :)

[othaime-en]

Hey @MasterofBlockchain the try--catch block is mainly useful because it lets us handle the error and terminate the program execution gracefully. When using the normal program flow, if an error is thrown the program abruptly ends and prints out the error. However when we use a try...catch block it gives us control on what to do when an error occurs. Say we don't want to see the error log, we could just say that if an error occurs prints a specific error message or a shortened version of the error or just a statement like an error occured. Additionally, we could do other things like run a specific function if a certain specific error occurs among other things. Using a try...catch gives us flexibility to continue program execution even if an error occurs since the we "capture" the error and do whatever we want with it.

Concerning the "listener" a listener is in simple terms a function that "listens" for or waits for a certain event to occur so that we can perform some actions. Javascript and Node.js in this case provides us with listerners that we can use. There is emitter.once() and emitter.on() The word emitter mainly just stands for whatever will emit this event we are listening for so in your case, we are using raffle.once() because the event comes from the raffle contract. emitter.on() is used when we want to continuously listen for an event being emmitted. Emitter.once() like in our case is used when we just want to respond to an event only one time (i.e. when the event is emitted the first time)
So in our case, we are setting up a listener on raffle using .once() we are saying that once the event we specify (in this case WinnerPicked ) is emitted, do whatever actions we have listed below. So when the raffle contract runs, we sit there patiently waiting for the WinnerPicked event and the first time the WinnerPicked event is emitted, we will perform the actions we want in the try...catch block below.

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listener definition is great! I have got it.

About try and catch there is some confusion.

I understand When using the normal program flow, if an error is thrown the program abruptly ends and prints out the error. However when we use a try...catch block it gives us control on what to do when an error occurs.

however we are not doing anything with try catch other than mentioning it. We have not said if this happens do this or show this error etc. just we are making sure our program runs smooth and if there is an error it gets caught. now back to the original question if its gets caught here it would be as same as regular program flow since we have not mentioned anything in try catch nah?

[othaime-en]

Yes. Right now, because there isn't much inside the try...catch the program will behave just like normal program execution and will just print out the error if it occurs. So it is up to us to decide whatever functionality we want to put inside the try...catch block to handle the error.

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Got it. So how it makes a difference to our program right now if it's gonna behave like regular function!?

Whats the usage of it if it being used as a regular one? I hope my questions makes sense.

[othaime-en]

It makes sense if we are going to expand the try...catch block and add more functionality to it. If we leave it as it is right now, then there is not much need for it because it just the same as regular program execution flow. But in the course, we added more functionality to it so of course it became useful to us.

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Thank you very much. Got it clearly.