Handling Errors | Lecture 144 | React.JS 🔥
Summary
TLDRThe video script focuses on error handling in web applications, particularly when dealing with asynchronous data fetching. It emphasizes the importance of being prepared for potential issues such as loss of internet connection or API errors. The tutorial demonstrates how to simulate and handle these scenarios by using try-catch blocks in JavaScript, updating application states to reflect errors, and displaying user-friendly error messages. It also introduces a state variable to manage error messages and conditional rendering to ensure a smooth user experience, even when data fetching fails.
Takeaways
- 🔍 Always assume errors can occur when dealing with asynchronous data in web applications.
- 💻 Simulate network issues like loss of internet connection to test error handling.
- 📶 Check the network tab to ensure the application behaves correctly under slow or offline conditions.
- 🛑 Use try-catch blocks to handle errors manually, as fetch does not inherently react to errors.
- 📌 Check the response object's `ok` property to determine if the response is successful or not.
- 🔁 Throw new errors when the response is not okay to handle specific error cases.
- 🔄 Use a finally block to ensure certain code is executed regardless of whether an error occurred.
- 📋 Maintain a separate state variable to track and display error messages.
- 📝 Log error messages to the console for debugging purposes.
- 🎨 Create a simple presentational component to display error messages with appropriate styling.
- 🔄 Ensure that the application's loading, error, and data display states are mutually exclusive.
Q & A
Why is it important to handle errors during data fetching in web applications?
-It is important to handle errors to provide a better user experience by informing users of issues, such as a lost internet connection, and to avoid leaving the application in a perpetual loading state.
How can you simulate a lost internet connection in a web application for testing purposes?
-You can simulate a lost internet connection by using the network tab in the browser's developer tools to set the connection to 'offline' while data is loading.
What does the 'response.ok' property signify in the context of fetch requests in JavaScript?
-The 'response.ok' property indicates whether the response was successful (status in the range 200-299) or not, helping to identify request failures.
What should you do in JavaScript code if the 'response.ok' property is false?
-If 'response.ok' is false, you should throw a new error to indicate that something went wrong with the fetch request.
What is the purpose of using a try-catch block in JavaScript?
-A try-catch block is used to catch errors in a section of code, allowing developers to handle exceptions and prevent the application from crashing.
Why is conditional rendering used in web applications?
-Conditional rendering is used to display different components or elements based on certain conditions, such as loading states or the presence of errors, enhancing user experience.
How can you manage the loading state in a React application to handle asynchronous data fetching?
-In a React application, you manage the loading state by using state variables to indicate when data is being fetched and then updating these variables appropriately to reflect when loading is complete or an error occurs.
What is the significance of the 'finally' block in error handling in JavaScript?
-The 'finally' block is executed after the try and catch blocks, regardless of the result, allowing for cleanup actions or resetting states like the loading state, ensuring code runs even after an error.
How can the absence of data from an API be handled in a web application?
-The absence of data can be handled by checking the received data and throwing an error or displaying a specific message if the expected data is undefined or does not meet required conditions.
Why is it important to handle different types of errors separately in web applications?
-Handling different types of errors separately allows for more tailored user feedback and specific troubleshooting steps, improving the overall user experience and application reliability.
Outlines
🔍 Error Handling in Data Fetching
This paragraph discusses the importance of error handling when fetching data in web applications, especially with asynchronous data. It highlights the need to account for scenarios such as users losing internet connection. The speaker simulates this by switching to slow 3G and then to offline mode, demonstrating how the application should not remain in a loading state indefinitely. The script explains how to handle errors by checking the response object from the fetch function and throwing a new error if the response is not okay. It also introduces the use of a try-catch block to handle errors and the creation of a new error component to display error messages on the screen.
📊 Conditional Rendering for Error Display
The second paragraph focuses on the implementation of conditional rendering to display error messages or movie lists based on the application's state. It explains the process of nesting ternary operators to manage different states such as loading, error, and no data. The speaker emphasizes the importance of ensuring that only one of the three mutually exclusive conditions (loading, error, or no data) is true at any given time to avoid displaying multiple components simultaneously. The paragraph also addresses the issue of the loading state remaining true even after an error is thrown and suggests using a finally block to ensure that the loading state is reset.
🛠️ Handling API Response Errors
The third paragraph delves into handling errors that arise from API responses, such as when no movies are found for a search query. It demonstrates how to check the API response for faults and throw a new error with a custom message like 'movie not found'. The speaker recaps the process of setting an error state and using it for conditional rendering on the UI. The paragraph concludes by stressing the significance of error handling in web applications, which is often overlooked but is essential for a smooth user experience.
Mindmap
Keywords
💡asynchronous data
💡error handling
💡fetch function
💡try-catch block
💡state management
💡conditional rendering
💡API response
💡React
💡finally block
💡ternary operator
💡presentational component
Highlights
Assuming errors in web applications dealing with asynchronous data is crucial.
Simulating a user losing internet connection to test error handling.
Using a try-catch block to handle errors in JavaScript.
Checking the response object's 'ok' property to determine if an error occurred.
Throwing a new error when the response is not okay.
Displaying an error message on the screen instead of keeping the application in a loading state.
Creating a new state variable to indicate the presence of an error.
Using conditional rendering to display the error message component when an error exists.
Creating an error message component to display error messages with an emoji.
Handling situations where the API response is undefined or an empty array.
Throwing an error when no movies are found for a search query.
Using a finally block to ensure the loading state is reset after an error.
Ensuring that the error, loading, and data display conditions are mutually exclusive.
Reacting to errors is not built into the fetch function and must be handled manually.
The importance of error handling in web applications, which is often overlooked but essential.
Analyzing the code after the video to understand the conditions and error handling logic.
Transcripts
So whenever we are doing any data fetching
in any web application
and dealing with asynchronous data,
we always need to assume that something can go wrong.
And so therefore, let's now account
for that situation by handling those errors.
So one of the things that can go wrong
is your users suddenly losing their internet connection.
So let's simulate that here again in our network tab
and let's make sure that we are first on slow 3G.
Then let's reload here, and then
while the movies are loaded, we will click on offline.
So right now,
so you see that now the application
basically never leaves the state.
And also when we come to our console here we see
that we failed to fetch,
which again is because our user basically now
lost their internet connection.
So when that happens, we want to display some kind
of error message here on the screen and not
keep the application in this loading state here forever.
Because like this, the user will think
that the data might eventually arrive
but of course it will not.
Now, reacting to errors like this is actually not built
into the fetch function itself.
And so we have to kind of do that manually.
And so let's try that here in our fetch movies function.
So here on the response object
that we receive from fetch exists one, okay, property.
And so here we can check for that.
So basically if the response is not okay
then we want to throw a new error.
And again, this is pretty standard JavaScript code.
So then let's just say something went wrong
with fetching movies now, okay?
And so now if we throw an error here
we need to wrap all of our code into a try catch block. So,
try and catch
which again is just a normal JavaScript feature.
This one has nothing to do with React.
Okay, and here let's console that error.
That error, okay?
And then let's come back here.
Well, if we load now, then of course nothing will work.
So let's do that trick again where first
we set it to slow 3G,
then let's reload and then let's wait for it.
We set our users back to offline right now.
And so where is it?
Well, it's not actually anywhere here,
but well, that's not really important anyway.
So let's actually just log error dot message.
So this is the property of the error
where this string here will get saved into
but then what we're actually interested in,
is to get this message here onto the screen.
So basically displaying it right here instead of loading.
So that means that we need another piece of state.
So basically a piece of state indicating
whether we currently have an error or not.
So const error
and set error.
And so here, this one is actually not a bullion,
but it's truly the error message.
Okay, so here
let's then set the error
to actually that message.
And again, error dot message is basically
this string that we passed into the error
and error is the error itself
as it was passed into this catch block.
And again, that is just basic JavaScript.
All right,
and then let's do some conditional rendering here
to basically get the error here
onto the screen whenever there exists one.
Now, okay, and let's start
by creating a new error component here
or maybe let's call it error message.
And it will receive a prop
with some message that it will then display on the screen.
So let's return again a paragraph this time
with the class name of error, which,
as always, I already included into my CSS file.
Then here let's maybe add some small emoji here,
like this one.
So showing there was some kind of problem.
And then here we simply display that message.
So this is a very simple presentational component.
Remember that?
Okay. Now here in our conditional rendering
basically what we want to do is
that when it's no longer loading
then we want to display this movie list
but only if there was no error.
So basically here we now would have to nest another ternary
inside this ternary, but that makes for really ugly coat.
And so let's do something else instead.
So let's comment out this entire part.
Or well, only this one.
So the box of course we still need,
but then let's do is loading.
And if there's no error
then display this movies list right here.
Now if there is an error
then display the error message component
with the error or I think it's called message.
So with the message prop set to error.
And here of course, it needs to be
that the data is not loading and there is no error.
And finally, we also need to account
of course for the is loading state.
So if it is loading, then just display our loader.
So here the situation is indeed a little bit tricky
with all these different states that we have
and with all the conditional rendering, but well
these are now three mutually exclusive conditions.
So it is either loading or it is not loading
and there is no error or there is an error.
So only one of these three here
can be true at the same time.
And so that's very important
so that we don't display multiple
of these components at the same time by mistake.
So let's give it a save and let's try again.
So back to our 3G
and then I will set it to offline very soon.
Let's wait for it.
And there it is.
So we get our error message correctly displayed here.
However, there's still some problem
because the loading state is still set to true
and in fact that's actually correct.
So as soon as the error here is thrown,
this rest of the code is no longer evaluated.
And so therefore is loading is never set to fault.
So our application will keep thinking
that the data is still being loaded.
So again, the problem is
that after this error is being thrown
then React will basically never see
this piece of code here,
where the loading state is reset.
So instead of doing that here
let's attach a finally block here.
So this block of code here will basically always be executed
at the very end.
Okay? And as an alternative
we could have kept the code here and also pasted it here,
but that would've created a duplication of code.
So with this, it's going to be a lot better.
Now, I will not try this yet here
because actually I want to handle another kind of error
which is not really an error,
but also situation where we want to display a message
which is the situation
in which we cannot find any movie for the search Query.
So let's say the Query is like this one here
and of course the API will not find anything.
So let's remove all throttling,
and well, what happens here?
Well, basically the length cannot be read of undefined.
And so the problem here is that the data
that comes back from the API now,
is apparently undefined.
So let's take a look at that.
So just at data here, and let's try that again.
And so indeed, we no longer have the search property here,
so we no longer have data dot search.
And so what's happening then is that data search
is being set to undefined,
and therefore we get this other error here.
So as I was saying in the very beginning
we always need to handle
all these different situations that can go wrong.
And when we are working with data fetching
there's always a lot of things that can go wrong.
So working with data is a lot of work
but it's also essential in most
if not all web applications.
But anyway, here we can now use this response
to our advantage,
in order to throw another error in this situation.
So we can say that data dot response,
and actually, this now needs to be here
after we already have the data.
So in this case, if data responds is equal faults
and so for some reason the API here responds
with the string of faults and not a bullion
but while this still works, so in this case, let's
also throw a new error and let's simply say
movie not found.
Alright, and beautiful.
Let's just reload.
And then here we get this log.
So that's this console dot error coming from here.
And then indeed we get our error message also displayed
on the UI.
So this one was maybe a little bit trickier.
So let's just quickly recap.
So what we did was to implement another state variable,
this time, specific for the error
so that whenever some error occurred
we could store the error message in there
and then display it in the UI as soon as an error occurred.
Now, as soon as an error did occur, which is
in this situation, and in this one we threw a new error,
and then we caught that error
inside the catch block of this try-catch.
And so this is a standard way
of catching errors in JavaScript.
And so in this situation, we then set the error state
to the message of the error that we specified here.
Then finally, we used of course, that state variable
in order to render something on the screen conditionally.
And so that was right here.
So this part here is maybe a bit confusing.
And so make sure that after this video you just
analyze exactly what's happening here and that,
in fact these three conditions here are mutually exclusive.
So only one of them will ever be true.
Okay? And I think that for now this is enough
for error handling in this application.
So that's always a very,
very important part that many people overlook.
But it is of course essential
to deal with these kind of situations.
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