js try catch assignment

Error handling, "try...catch"

No matter how great we are at programming, sometimes our scripts have errors. They may occur because of our mistakes, an unexpected user input, an erroneous server response, and for a thousand other reasons.

Usually, a script “dies” (immediately stops) in case of an error, printing it to console.

But there’s a syntax construct try...catch that allows us to “catch” errors so the script can, instead of dying, do something more reasonable.

The “try…catch” syntax

The try...catch construct has two main blocks: try , and then catch :

It works like this:

• First, the code in try {...} is executed.
• If there were no errors, then catch (err) is ignored: the execution reaches the end of try and goes on, skipping catch .
• If an error occurs, then the try execution is stopped, and control flows to the beginning of catch (err) . The err variable (we can use any name for it) will contain an error object with details about what happened.

So, an error inside the try {...} block does not kill the script – we have a chance to handle it in catch .

Let’s look at some examples.

An errorless example: shows alert (1) and (2) :

An example with an error: shows (1) and (3) :

For try...catch to work, the code must be runnable. In other words, it should be valid JavaScript.

It won’t work if the code is syntactically wrong, for instance it has unmatched curly braces:

The JavaScript engine first reads the code, and then runs it. The errors that occur on the reading phase are called “parse-time” errors and are unrecoverable (from inside that code). That’s because the engine can’t understand the code.

So, try...catch can only handle errors that occur in valid code. Such errors are called “runtime errors” or, sometimes, “exceptions”.

If an exception happens in “scheduled” code, like in setTimeout , then try...catch won’t catch it:

That’s because the function itself is executed later, when the engine has already left the try...catch construct.

To catch an exception inside a scheduled function, try...catch must be inside that function:

Error object

When an error occurs, JavaScript generates an object containing the details about it. The object is then passed as an argument to catch :

For all built-in errors, the error object has two main properties:

There are other non-standard properties available in most environments. One of most widely used and supported is:

For instance:

Optional “catch” binding

If we don’t need error details, catch may omit it:

Using “try…catch”

Let’s explore a real-life use case of try...catch .

As we already know, JavaScript supports the JSON.parse(str) method to read JSON-encoded values.

Usually it’s used to decode data received over the network, from the server or another source.

We receive it and call JSON.parse like this:

You can find more detailed information about JSON in the JSON methods, toJSON chapter.

If json is malformed, JSON.parse generates an error, so the script “dies”.

Should we be satisfied with that? Of course not!

This way, if something’s wrong with the data, the visitor will never know that (unless they open the developer console). And people really don’t like when something “just dies” without any error message.

Let’s use try...catch to handle the error:

Here we use the catch block only to show the message, but we can do much more: send a new network request, suggest an alternative to the visitor, send information about the error to a logging facility, … . All much better than just dying.

Throwing our own errors

What if json is syntactically correct, but doesn’t have a required name property?

Here JSON.parse runs normally, but the absence of name is actually an error for us.

To unify error handling, we’ll use the throw operator.

“Throw” operator

The throw operator generates an error.

The syntax is:

Technically, we can use anything as an error object. That may be even a primitive, like a number or a string, but it’s better to use objects, preferably with name and message properties (to stay somewhat compatible with built-in errors).

JavaScript has many built-in constructors for standard errors: Error , SyntaxError , ReferenceError , TypeError and others. We can use them to create error objects as well.

Their syntax is:

For built-in errors (not for any objects, just for errors), the name property is exactly the name of the constructor. And message is taken from the argument.

Let’s see what kind of error JSON.parse generates:

As we can see, that’s a SyntaxError .

And in our case, the absence of name is an error, as users must have a name .

So let’s throw it:

In the line (*) , the throw operator generates a SyntaxError with the given message , the same way as JavaScript would generate it itself. The execution of try immediately stops and the control flow jumps into catch .

Now catch became a single place for all error handling: both for JSON.parse and other cases.

In the example above we use try...catch to handle incorrect data. But is it possible that another unexpected error occurs within the try {...} block? Like a programming error (variable is not defined) or something else, not just this “incorrect data” thing.

For example:

Of course, everything’s possible! Programmers do make mistakes. Even in open-source utilities used by millions for decades – suddenly a bug may be discovered that leads to terrible hacks.

In our case, try...catch is placed to catch “incorrect data” errors. But by its nature, catch gets all errors from try . Here it gets an unexpected error, but still shows the same "JSON Error" message. That’s wrong and also makes the code more difficult to debug.

To avoid such problems, we can employ the “rethrowing” technique. The rule is simple:

Catch should only process errors that it knows and “rethrow” all others.

The “rethrowing” technique can be explained in more detail as:

• Catch gets all errors.
• In the catch (err) {...} block we analyze the error object err .
• If we don’t know how to handle it, we do throw err .

Usually, we can check the error type using the instanceof operator:

We can also get the error class name from err.name property. All native errors have it. Another option is to read err.constructor.name .

In the code below, we use rethrowing so that catch only handles SyntaxError :

The error throwing on line (*) from inside catch block “falls out” of try...catch and can be either caught by an outer try...catch construct (if it exists), or it kills the script.

So the catch block actually handles only errors that it knows how to deal with and “skips” all others.

The example below demonstrates how such errors can be caught by one more level of try...catch :

Here readData only knows how to handle SyntaxError , while the outer try...catch knows how to handle everything.

try…catch…finally

Wait, that’s not all.

The try...catch construct may have one more code clause: finally .

If it exists, it runs in all cases:

• after try , if there were no errors,
• after catch , if there were errors.

The extended syntax looks like this:

Try running this code:

The code has two ways of execution:

• If you answer “Yes” to “Make an error?”, then try -> catch -> finally .
• If you say “No”, then try -> finally .

The finally clause is often used when we start doing something and want to finalize it in any case of outcome.

For instance, we want to measure the time that a Fibonacci numbers function fib(n) takes. Naturally, we can start measuring before it runs and finish afterwards. But what if there’s an error during the function call? In particular, the implementation of fib(n) in the code below returns an error for negative or non-integer numbers.

The finally clause is a great place to finish the measurements no matter what.

Here finally guarantees that the time will be measured correctly in both situations – in case of a successful execution of fib and in case of an error in it:

You can check by running the code with entering 35 into prompt – it executes normally, finally after try . And then enter -1 – there will be an immediate error, and the execution will take 0ms . Both measurements are done correctly.

In other words, the function may finish with return or throw , that doesn’t matter. The finally clause executes in both cases.

Please note that result and diff variables in the code above are declared before try...catch .

Otherwise, if we declared let in try block, it would only be visible inside of it.

The finally clause works for any exit from try...catch . That includes an explicit return .

In the example below, there’s a return in try . In this case, finally is executed just before the control returns to the outer code.

The try...finally construct, without catch clause, is also useful. We apply it when we don’t want to handle errors here (let them fall through), but want to be sure that processes that we started are finalized.

In the code above, an error inside try always falls out, because there’s no catch . But finally works before the execution flow leaves the function.

Global catch

The information from this section is not a part of the core JavaScript.

Let’s imagine we’ve got a fatal error outside of try...catch , and the script died. Like a programming error or some other terrible thing.

Is there a way to react on such occurrences? We may want to log the error, show something to the user (normally they don’t see error messages), etc.

There is none in the specification, but environments usually provide it, because it’s really useful. For instance, Node.js has process.on("uncaughtException") for that. And in the browser we can assign a function to the special window.onerror property, that will run in case of an uncaught error.

The syntax:

The role of the global handler window.onerror is usually not to recover the script execution – that’s probably impossible in case of programming errors, but to send the error message to developers.

There are also web-services that provide error-logging for such cases, like https://errorception.com or https://www.muscula.com .

They work like this:

• We register at the service and get a piece of JS (or a script URL) from them to insert on pages.
• That JS script sets a custom window.onerror function.
• When an error occurs, it sends a network request about it to the service.
• We can log in to the service web interface and see errors.

The try...catch construct allows to handle runtime errors. It literally allows to “try” running the code and “catch” errors that may occur in it.

There may be no catch section or no finally , so shorter constructs try...catch and try...finally are also valid.

Error objects have following properties:

• message – the human-readable error message.
• name – the string with error name (error constructor name).
• stack (non-standard, but well-supported) – the stack at the moment of error creation.

If an error object is not needed, we can omit it by using catch { instead of catch (err) { .

We can also generate our own errors using the throw operator. Technically, the argument of throw can be anything, but usually it’s an error object inheriting from the built-in Error class. More on extending errors in the next chapter.

Rethrowing is a very important pattern of error handling: a catch block usually expects and knows how to handle the particular error type, so it should rethrow errors it doesn’t know.

Even if we don’t have try...catch , most environments allow us to setup a “global” error handler to catch errors that “fall out”. In-browser, that’s window.onerror .

Finally or just the code?

Compare the two code fragments.

The first one uses finally to execute the code after try...catch :

The second fragment puts the cleaning right after try...catch :

We definitely need the cleanup after the work, doesn’t matter if there was an error or not.

Is there an advantage here in using finally or both code fragments are equal? If there is such an advantage, then give an example when it matters.

The difference becomes obvious when we look at the code inside a function.

The behavior is different if there’s a “jump out” of try...catch .

For instance, when there’s a return inside try...catch . The finally clause works in case of any exit from try...catch , even via the return statement: right after try...catch is done, but before the calling code gets the control.

…Or when there’s a throw , like here:

It’s finally that guarantees the cleanup here. If we just put the code at the end of f , it wouldn’t run in these situations.

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• try...catch

The finally clause

Returning from a finally block.

The try...catch statement marks a block of statements to try, and specifies a response, should an exception be thrown.

Description

The try statement consists of a try block, which contains one or more statements ( {} must always be used, also for single statements), and at least one catch clause or a finally clause, or both. That is, there are three forms of the try statement:

• try...finally
• try...catch...finally

A catch clause contains statements that specify what to do if an exception is thrown in the try block. That is, you want the try block to succeed, and if it does not succeed, you want control to pass to the catch block. If any statement within the try block (or in a function called from within the try block) throws an exception, control immediately shifts to the catch clause. If no exception is thrown in the try block, the catch clause is skipped.

The finally clause executes after the try block and catch clause(s) execute but before the statements following the try statement. It always executes, regardless of whether or not an exception was thrown or caught.

You can nest one or more try statements. If an inner try statement does not have a catch clause, the enclosing try statement's catch clause is entered.

You also use the try statement to handle JavaScript exceptions. See the JavaScript Guide for more information on JavaScript exceptions.

Unconditional catch clause

When a single, unconditional catch clause is used, the catch block is entered when any exception is thrown. For example, when the exception occurs in the following code, control transfers to the catch clause.

The  catch  block specifies an identifier ( e  in the example above) that holds the value specified by the  throw  statement. The catch block is unique in that JavaScript creates this identifier when the  catch  block is entered and it adds it to the current scope; the identifier lasts only for the duration of the  catch  block; after the  catch  block finishes executing, the identifier is no longer available.

Conditional catch clauses

Non-standard This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

You can also use one or more conditional catch clauses to handle specific exceptions. In this case, the appropriate catch clause is entered when the specified exception is thrown. In the following example, code in the try block can potentially throw three exceptions: TypeError , RangeError , and EvalError . When an exception occurs, control transfers to the appropriate catch clause. If the exception is not one of the specified exceptions and an unconditional catch clause is found, control transfers to that catch clause.

If you use an unconditional catch clause with one or more conditional catch clauses, the unconditional catch clause must be specified last. Otherwise, the unconditional catch clause will intercept all types of exception before they can reach the conditional ones.

Reminder: this functionality is not part of the ECMAScript specification.

And here is how to do implement the same "Conditional catch clauses" using only simple JavaScript conforming to the ECMAScript specification (obviously it's more verbose, but works everywhere):

The exception identifier

When an exception is thrown in the try block, exception_var (e.g. the e in catch (e) ) holds the value specified by the throw statement. You can use this identifier to get information about the exception that was thrown.

This identifier is local to the catch clause. That is, it is created when the catch clause is entered, and after the catch clause finishes executing, the identifier is no longer available.

The finally clause contains statements to execute after the try block and catch clause(s) execute, but before the statements following the try..catch..finally  block. Note that the finally clause executes regardless of whether or not an exception is thrown. Also, if an exception is thrown, the statements in the finally clause execute even if no catch clause handles the exception.  You can use the finally clause to make your script fail gracefully when an exception occurs; for example, to do general cleanup, you may need to release a resource that your script has tied up.

It may seem strange to have a special exception-related clause that executes REGARDLESS of whether there is an exception, but this construct actually does serve a purpose.  The important point, though, is not that the finally-clause always executes, but rather that ordinary code otherwise following a try..catch, does not.  For instance, if another exception occurs inside a try's catch-block, any remaining code in the same outer try-block enclosing that try..catch (or in the main flow, if not in an outer try-block) , will not get executed, since control is immediately transferred to the outer try's catch-block (or the internal error-generator, if not in a try-block).  Thus, any routine cleanup code done in that enclosed (or the main) section before it exits, will be skipped.  However, If the try-block has a finally-block, then that finally-block code will be executed first to permit any such cleanup, and THEN the other try's catch-block (or the error-generator) will get control to handle the second exception.  Now, if that routine cleanup must be done whether or not the try..catch code succeeds, then if the finally-block only executed after an exception, the same cleanup code would have to be duplicated both inside and outside the finally-block, and therefore there is no reason not to have just the finally-block alone, and let it execute regardless of exceptions or not.

The following example opens a file and then executes statements that use the file (server-side JavaScript allows you to access files). If an exception is thrown while the file is open, the finally clause closes the file before the script fails. The code in finally also executes upon explicitly returning from try or catch block.

Nested try-blocks

First let's see what happens with this:

Now, if we already caught the exception in the inner try-block by adding a catch block

And now, lets re-throw the error.

Any given exception will be caught only once by the nearest enclosing catch-block, unless it is re-thrown. Of course, any new exceptions raised in the "inner" block (because code in catch-block may do something that throws), will be caught by the "outer" block.

If the finally block returns a value, this value becomes the return value of the entire try-catch-finally production, regardless of any return statements in the try and catch blocks. This includes exceptions thrown inside of the catch block:

The outer "oops" is not thrown because of the return in the finally block. The same would apply to any value returned from the catch block.

Specifications

Browser compatibility, document tags and contributors.

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try...catch: The Right Way

Posted on 28 April 2019 , by Markus Oberlehner , in Development , tagged JavaScript

Because of a recent bug hunting session where it took me a couple of hours to drill down several levels of npm dependencies in order to finally find a try...catch statement with the catch part being empty, I decided to write an article about this topic. Although, I have to admit, my first instinct was to be angry at the developer who did this, I figured taking a closer look at how to properly deal with exceptions in Node.js and JavaScript would be a better way to deal with the situation. I also realized that I myself am not entirely innocent in this respect.

Good reasons for an empty catch block?

There are situations where, just like the developer who caused my several hours of debugging, we might think: I know that in certain situations this could throw an error, but actually I’m okay with that and it is not really an error. One example for this, and a situation in which I myself have been guilty of writing an empty catch statement, is trying to require a custom config file.

If there is a custom config file, we want to load it, but if not that’s fine too. In the case there is no custom config we simply stay with the defaults. Of course we could check if the file exists before trying to require it, but why make the code more complicated?

So what’s the problem with using an empty catch block in this situation? The problem is that we catch all errors although we don’t really know if all the other errors which might occur in the try block are actually unimportant for our code to run correctly.

If we take a look at the example above requiring this config file would throw a TypeError with the message Assignment to constant variable . But because we intercept all errors, our code would continue with the default configuration and our user would wonder why their custom configuration is not loaded correctly.

By checking the error code we can make sure to only suppress errors which we actually don’t care about like the MODULE_NOT_FOUND error code in our example. But if any other error happens we throw the error to end execution.

Silently handling errors

This excellent article differentiates between two kinds of errors: operational errors and programmer errors. Operational errors are basically errors where our code works correctly but something outside of our control (e.g. an API request) does fail. Programmer errors on the other hand are unknown bugs in our code.

In case of operational errors we can basically decide between three strategies.

• Catch the error and retry the failed operation.
• Catch the error and fail silently or show the user an error message they can easily understand.
• Don’t catch the error at all or throw a custom error.

If we deal with a network request we could look for certain error codes and retry the operation a couple of times before we actually throw an error.

Also, especially in the case of certain network errors it could absolutely make sense to catch the error and either fail silently (if the failing API request is not crucial to the functionality of our application) or show the user a nice error message.

In the example above you can see how we could deal with network errors in a non critical part of our application. It isn’t a big deal if the user does not see all of their latest notifications at any given time because our notification microservice is down for some reason. Moreover, network errors are usually not the fault of our application but operational errors. Nevertheless, we want to keep an eye on them so we can use a tool like Sentry to log those errors but we set the error level to info instead of error .

Not handling errors

Depending how low level the code you write is, you can also decide do not handle the error yourself but let the consumer code decide how it deals with errors. As a general rule of thumb, error handling should always happen at the highest level. So if you have an article-service.js file which contains functions which make API requests to fetch articles and you also have several places in your code where you use this service, you most likely don’t want to catch errors in article-service.js but in the places where you actually use the service.

Instead of catching the error directly in the article service, where we wouldn’t really know what to do with it anyway, we capture it where we actually use the article service. Here we can decide either to render an error message or to handle the error silently, e.g. if we only render a few article teasers in a sidebar, which isn’t important at all for the user.

How to really deal with unhandled promise rejection in Node.js

One thing that many of you have certainly stumbled upon is the infamous, UnhandledPromiseRejectionWarning in Node.js.

If you encounter this warning and you look on Stack Overflow for solutions, most examples you will find show how to catch the error and they add a simple console.log() in the catch block, but that’s not always a good solution to deal with this situation. What if you actually want to stop execution in case a promise rejects?

Above you can see the best solution I could find in order to deal with such situations in a way which not also makes Node.js happy and gets rid of the DeprecationWarning but also stops further execution of the script.

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Wrapping it up

If there is only one thing you’ve learned from this article, I hope it is that you never should have an empty catch block in your code. Yes, in some situations it might make sense to catch certain errors, but I can’t think of a situation where you definitely want to catch all errors no matter what.

• Joyent, Error Handling in Node.js
• Dr. Axel Rauschmayer, Promise-based functions should not throw exceptions

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Try/Catch in JavaScript – How to Handle Errors in JS

Bugs and errors are inevitable in programming. A friend of mine calls them unknown features :).

Call them whatever you want, but I honestly believe that bugs are one of the things that make our work as programmers interesting.

I mean no matter how frustrated you might be trying to debug some code overnight, I am pretty sure you will have a good laugh when you find out that the problem was a simple comma you overlooked, or something like that. Although, an error reported by a client will bring about more of a frown than a smile.

That said, errors can be annoying and a real pain in the behind. That is why in this article, I want to explain something called try / catch in JavaScript.

What is a try/catch block in JavaScript?

A try / catch block is basically used to handle errors in JavaScript. You use this when you don't want an error in your script to break your code.

While this might look like something you can easily do with an if statement , try/catch gives you a lot of benefits beyond what an if/else statement can do, some of which you will see below.

A try statement lets you test a block of code for errors.

A catch statement lets you handle that error. For example:

This is basically how a try/catch is constructed. You put your code in the try block , and immediately if there is an error, JavaScript gives the catch statement control and it just does whatever you say. In this case, it alerts you to the error.

All JavaScript errors are actually objects that contain two properties: the name (for example, Error, syntaxError, and so on) and the actual error message. That is why when we alert e , we get something like ReferenceError: getData is not defined .

Like every other object in JavaScript, you can decide to access the values differently, for example e.name (ReferenceError) and e.message (getData is not defined).

But honestly this is not really different from what JavaScript will do. Although JavaScript will respect you enough to log the error in the console and not show the alert for the whole world to see :).

What, then, is the benefit of try/catch statements?

How to use try/catch statements

The throw statement.

One of the benefits of try/catch is its ability to display your own custom-created error. This is called (throw error) .

In situations where you don't want this ugly thing that JavaScript displays, you can throw your error (an exception) with the use of the throw statement . This error can be a string, boolean, or object. And if there is an error, the catch statement will display the error you throw.

This is nice, right? But we can take it a step further by actually throwing an error with the JavaScript constructor errors.

Basically JavaScript categorizes errors into six groups:

• EvalError - An error occurred in the eval function.
• RangeError - A number out of range has occurred, for example 1.toPrecision(500) . toPrecision basically gives numbers a decimal value, for example 1.000, and a number cannot have 500 of that.
• ReferenceError -  Using a variable that has not been declared
• syntaxError - When evaluating a code with a syntax error
• TypeError - If you use a value that is outside the range of expected types: for example 1.toUpperCase()
• URI (Uniform Resource Identifier) Error - A URIError is thrown if you use illegal characters in a URI function.

So with all this, we could easily throw an error like throw new Error("Hi there") . In this case the name of the error will be Error and the message Hi there . You could even go ahead and create your own custom error constructor, for example:

And you can easily use this anywhere with throw new CustomError("data is not defined") .

So far we have learnt about try/catch and how it prevents our script from dying, but that actually depends. Let's consider this example:

But when you try it out, even with the try statement, it still does not work. This is because there are two main types of errors in JavaScript (what I described above –syntaxError and so on – are not really types of errors. You can call them examples of errors): parse-time errors and runtime errors or exceptions .

Parse-time errors are errors that occur inside the code, basically because the engine does not understand the code.

For example, from above, JavaScript does not understand what you mean by {{}} , and because of that, your try / catch has no use here (it won't work).

On the other hand, runtime errors are errors that occur in valid code, and these are the errors that try/catch will surely find.  

Believe it or not, the above is valid code and the try /catch will handle the error appropriately.

The Finally statement

The finally statement acts like neutral ground, the base point or the final ground for your try/ catch block. With finally, you are basically saying no matter what happens in the try/catch (error or no error), this code in the finally statement should run . For example:

Nesting try blocks

You can also nest try blocks, but like every other nesting in JavaScript (for example if, for, and so on), it tends to get clumsy and unreadable, so I advise against it. But that is just me.

Nesting try blocks gives you the advantage of using just one catch statement for multiple try statements. Although you could also decide to write a catch statement for each try block, like this:

In this case, there won't be any error from the outer try block because nothing is wrong with it. The error comes from the inner try block, and it is already taking care of itself (it has it own catch statement). Consider this below:

This code above works a little bit differently: the error occurs in the inner try block with no catch statement but instead with a finally statement.

Note that try/catch can be written in three different ways: try...catch , try...finally , try...catch...finally ), but the error is throw from this inner try.

The finally statement for this inner try will definitely work, because like we said earlier, it works no matter what happens in try/catch. But even though the outer try does not have an error, control is still given to its catch to log an error. And even better, it uses the error we created in the inner try statement because the error is coming from there.

If we were to create an error for the outer try, it would still display the inner error created, except the inner one catches its own error.

You can play around with the code below by commenting out the inner catch.

The Rethrow Error

The catch statement actually catches all errors that come its way, and sometimes we might not want that. For example,

Let's assume for a second that the number inputted will be less than 5 (the purpose of "use strict" is to indicate that the code should be executed in "strict mode"). With strict mode , you can not, for example, use undeclared variables ( source ).

I want the try statement to throw an error of y is not... when the value of y is greater than 5 which is close to impossible. The error above should be for y is not less... and not y is undefined .

In situations like this, you can check for the name of the error, and if it is not what you want, rethrow it :

This will simply rethrow the error for another try statement to catch or break the script here. This is useful when you want to only monitor a particular type of error and other errors that might occur as a result of negligence should break the code.

In this article, I have tried to explain the following concepts relating to try/catch:

• What try /catch statements are and when they work
• How to throw custom errors
• What the finally statement is and how it works
• How Nesting try / catch statements work
• How to rethrow errors

Thank you for reading. Follow me on twitter @fakoredeDami .

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Try...catch, Asynchronous JavaScript - promise, async, await, event loop ...

Destructuring assignment.

Destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables. Destructuring assignment -mdn

More about destructuring assignment -javascript.info

try...catch

When executing JavaScript code, different errors can occur. try...catch statement marks a block of statements to try and specifies a response should an exception be thrown. The try statement allows you to define a block of code to be tested for errors while it is being executed . The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

At above code, the first and third alerts will be executed only. The second alert won't be executed because lalala is not defined variable which occurs an error . Therefore, the second will be skipped and then go to catch and will execute the third alert.

JavaScript has many built-in constructors for standard errors: Error, SyntaxError, ReferenceError, TypeError and others.

try...catch...finally

If finally exists, in runs always no matter what.

How to use try...catch

More about try..catch -javascript.info

Promise is like a promise that you make that you don't know when you are going to call it but will call it no matter what. There are only success(completion) and failure for the result. The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. A Promise is in one of these states:

• pending : initial state, neither fulfilled nor rejected.
• fulfilled : meaning that the operation was completed successfully.
• rejected : meaning that the operation failed.

A Promise object serves as a link between the executor and the consuming functions, which will receive the result or error. Consuming functions can be registered (subscribed) using methods .then, .catch and .finally.

. then : then will be executed when previous code is resolved.

Promise - finally

About Promise -javascript.info Promise chaining

The Fetch API provides an interface for fetching resources (including across the network). It will seem familiar to anyone who has used XMLHttpRequest, but the new API provides a more powerful and flexible feature set.

About fetch

HTTP status codes

• 1xx informational response – the request was received, continuing process
• 2xx successful – the request was successfully received, understood, and accepted
• 3xx redirection – further action needs to be taken in order to complete the request
• 4xx client error – the request contains bad syntax or cannot be fulfilled
• 5xx server error – the server failed to fulfil an apparently valid request ( https://en.wikipedia.org/wiki/List_of_HTTP_status_codes )

async, await

An async function is a function declared with the async keyword, and the await keyword is permitted within them. The async and await keywords enable asynchronous, promise-based behavior to be written in a cleaner style, avoiding the need to explicitly configure promise chains.

Great video to watch to understand Event loop and asynchronous JavaScript

call stack / stack

JavaScript has single thread runtime which means it has a single call stack . So it can do one that at a time . and stack is like haystack. A horse will start eating what's on the bottom. For stacks, the latest pushed item is received first , that’s also called LIFO ( Last-In-First-Out ) principle.

You can implement this by using push and pop with Array/Object. The push method will add any object to the top of the stack and the pop method will remove it.

For queues, we have FIFO (First-In-First-Out). A queue is one of the most common uses of an array. enqueue —This operation is responsible for inserting or pushing a new element to the queue. dequeue —This operation is responsible for removing the oldest element from the queue. At some point during the event loop, the runtime starts handling the messages on the queue, starting with the oldest one. To do so, the message is removed from the queue and its corresponding function is called with the message as an input parameter. As always, calling a function creates a new stack frame for that function's use. The processing of functions continues until the stack is once again empty. Then, the event loop will process the next message in the queue (if there is one).

Objects are allocated in a heap which is just a name to denote a large (mostly unstructured) region of memory.

Event loop -mdn

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Javascript Try Catch

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Table of Contents

Introduction

Understanding try catch, working with try catch, tips and tricks, common error-prone cases.

• Javascript is a powerful, object-oriented scripting language that is used to make web pages interactive. One of the critical aspects of writing efficient Javascript code involves handling errors effectively. This is where the concept of 'Try Catch' comes into play.
• In Javascript, the try-catch statement is used to handle exceptions. It allows us to write a block of code that might throw an exception and catch that exception if it occurs. Let's dive deeper into understanding the try-catch statement and how we can use it in our Javascript code.
• The try-catch statement in Javascript is composed of two main parts: the 'try' block and the 'catch' block. The 'try' block contains the code that may potentially throw an exception. If an exception does occur, the 'catch' block is executed.
• Let's look at a simple example to understand how try-catch works.
• In this example, the line of code within the try block is trying to access an element at index '5' of the array 'numbers'. However, this index does not exist in the array, so an exception is thrown. The catch block catches this exception and logs the error message to the console.
• Always use the 'try' block around the code that might throw an exception. This way, if an exception occurs, it will be caught and handled gracefully instead of causing the program to crash.
• Remember to include a 'catch' block for every 'try' block. If you leave out the 'catch' block, the error will not be caught and the program may crash.
• It's good practice to log the error message in the 'catch' block. This way, you can debug the issue if an exception is thrown.
• One common error-prone case in Javascript involves trying to access a property of an object that does not exist. This will throw an exception. To avoid this, always check if the property exists before trying to access it.
• In this example, we are trying to log the 'grade' property of the 'student' object. But the 'grade' property does not exist, so an exception is thrown. The catch block catches this exception and logs the error message.
• Another common mistake is forgetting to include the 'catch' block after the 'try' block. Remember, a 'try' block must always be followed by a 'catch' block.
• The try-catch statement in Javascript is a powerful tool for handling exceptions. It allows us to write a block of code that might throw an exception and catch that exception if it occurs. By using try-catch properly, we can ensure that our Javascript code is robust and resilient to errors.

Home » JavaScript Tutorial » JavaScript try…catch…finally

JavaScript try…catch…finally

Summary : in this tutorial, you’ll learn how to use the JavaScript try...catch...finally statement to catch exceptions and execute a block whether the exceptions occur or not

Introduction to the JavaScript try…catch…finally statement

The try...catch statement allows you to catch exceptions and handle them gracefully. Sometimes, you want to execute a block whether exceptions occur or not. In this case, you can use the try...catch...finally statement with the following syntax:

In this syntax, the finally block always executes after the try and catch blocks complete and whether exceptions occur or not.

The following flowchart illustrates how the try...catch...finally works:

JavaScript try…catch…finally statement example

The following example illustrates how to use the try...catch...finally statement:

How it works.

First, declare the result variable and initialize its value with 0 .

Second, call the add() function and assign the return value to the result variable in the try block. Because the add() function does not exist, the JavaScript engine raises an exception.

Because of the exception, the statement in the catch block executes to show the error message.

Third, output the result to the console in the try block.

In the following example, we define the add() function and call it in the try block:

Because the add() function exists, no exception occurs in the try block. Therefore, the finally block outputs the value of the result variable, which is the sum of 10 and 20 .

In both examples, the finally block always runs.

try…catch…finally and return

The finally block always executes whether exceptions occur or not. Also, you can do nothing to prevent it from executing including using a return statement. For example:

In this example, the return statement in the try block returns 1 . Hence, the fn() function should have returned 1 . However, it is not the case.

Because the finally block always executes, the return statement in the finally block returns 3 . Therefore, the fn() function returns 3 .

In other words, the return statements in the try and catch blocks are ignored in the try...catch...finally statement.

• Use the finally clause in the try...catch...finally statement to execute a block whether exceptions occur or not.
• The try...catch...finally statement ignores the return statement in the try and catch blocks because the finally block always executes.

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JavaScript try...catch...finally Statement

The try , catch and finally blocks are used to handle exceptions (a type of an error). Before you learn about them, you need to know about the types of errors in programming.

• Types of Errors

In programming, there can be two types of errors in the code:

Syntax Error : Error in the syntax. For example, if you write consol.log('your result'); , the above program throws a syntax error. The spelling of  console is a mistake in the above code.

Runtime Error : This type of error occurs during the execution of the program. For example, calling an invalid function or a variable.

These errors that occur during runtime are called exceptions . Now, let's see how you can handle these exceptions.

• JavaScript try...catch Statement

The try...catch statement is used to handle the exceptions. Its syntax is:

The main code is inside the try block. While executing the try block, if any error occurs, it goes to the catch block. The catch block handles the errors as per the catch statements.

If no error occurs, the code inside the try block is executed and the catch block is skipped.

Example 1: Display Undeclared Variable

In the above program, a variable is not defined. When you try to print the a variable, the program throws an error. That error is caught in the catch block.

JavaScript try...catch...finally Statement

You can also use the try...catch...finally statement to handle exceptions. The finally block executes both when the code runs successfully or if an error occurs.

The syntax of try...catch...finally block is:

Example 2: try...catch...finally Example

In the above program, an error occurs and that error is caught by the catch block. The finally block will execute in any situation ( if the program runs successfully or if an error occurs).

Note : You need to use catch or finally statement after try statement. Otherwise, the program will throw an error Uncaught SyntaxError: Missing catch or finally after try .

JavaScript try...catch in setTimeout

The try...catch won't catch the exception if it happened in " timed " code, like in setTimeout() . For example,

The above try...catch won't work because the engine has already left the try..catch construct and the function is executed later.

The try..catch block must be inside that function to catch an exception inside a timed function. For example,

You can also use the throw statement with the try...catch statement to use user-defined exceptions. For example, a certain number is divided by 0 . If you want to consider Infinity as an error in the program, then you can throw a user-defined exception using the throw statement to handle that condition.

Table of Contents

• Introduction
• try...catch...finally Statement
• try...catch in setTimeout

Sorry about that.

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The try statement defines a code block to run (to try).

The catch statement defines a code block to handle any error.

The finally statement defines a code block to run regardless of the result.

The throw statement defines a custom error.

Errors Will Happen!

When executing JavaScript code, different errors can occur.

Errors can be coding errors made by the programmer, errors due to wrong input, and other unforeseeable things.

In this example we misspelled "alert" as "adddlert" to deliberately produce an error:

JavaScript catches adddlert as an error, and executes the catch code to handle it.

JavaScript try and catch

The try statement allows you to define a block of code to be tested for errors while it is being executed.

The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

The JavaScript statements try and catch come in pairs:

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JavaScript Throws Errors

When an error occurs, JavaScript will normally stop and generate an error message.

The technical term for this is: JavaScript will throw an exception (throw an error) .

JavaScript will actually create an Error object with two properties: name and message .

The throw Statement

The throw statement allows you to create a custom error.

Technically you can throw an exception (throw an error) .

The exception can be a JavaScript String , a Number , a Boolean or an Object :

If you use throw together with try and catch , you can control program flow and generate custom error messages.

Input Validation Example

This example examines input. If the value is wrong, an exception (err) is thrown.

The exception (err) is caught by the catch statement and a custom error message is displayed:

HTML Validation

The code above is just an example.

Modern browsers will often use a combination of JavaScript and built-in HTML validation, using predefined validation rules defined in HTML attributes:

You can read more about forms validation in a later chapter of this tutorial.

The finally Statement

The finally statement lets you execute code, after try and catch, regardless of the result:

The Error Object

JavaScript has a built in error object that provides error information when an error occurs.

The error object provides two useful properties: name and message.

Error Object Properties

Error name values.

Six different values can be returned by the error name property:

The six different values are described below.

An EvalError indicates an error in the eval() function.

Newer versions of JavaScript do not throw EvalError. Use SyntaxError instead.

Range Error

A RangeError is thrown if you use a number that is outside the range of legal values.

For example: You cannot set the number of significant digits of a number to 500.

Reference Error

A ReferenceError is thrown if you use (reference) a variable that has not been declared:

Syntax Error

A SyntaxError is thrown if you try to evaluate code with a syntax error.

A TypeError is thrown if an operand or argument is incompatible with the type expected by an operator or function.

URI (Uniform Resource Identifier) Error

A URIError is thrown if you use illegal characters in a URI function:

Non-Standard Error Object Properties

Mozilla and Microsoft define some non-standard error object properties:

fileName (Mozilla) lineNumber (Mozilla) columnNumber (Mozilla) stack (Mozilla) description (Microsoft) number (Microsoft)

Do not use these properties in public web sites. They will not work in all browsers.

Complete Error Reference

For a complete reference of the Error object, go to our Complete JavaScript Error Reference .

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Try Catch and Throw Errors in JavaScript (with code)

When dealing with some code JavaScript, we encounter several errors that need to be handled or removed. Errors occur for various reasons such as accessing an element out of bounds of the index. We need a proper way to deal with these errors to remove inconsistencies. Javascript throw statements or try-catch statements are some of the techniques that one can use to handle errors.

Try-Catch Block in JavaScript

The try-catch blocks in JavaScript are used to capture the errors and handle them. The try block encloses a piece of code that is doubtful of containing an error. The catch block is used to capture or catch that error to make further operations.

This simple mechanism allows developers to handle errors easily. The try block contains the possible problematic code that might cause an error during execution. If an exception occurs, the normal flow of the program is interrupted, and control is transferred to the associated catch block. The catch block contains the code that will be executed if such occurs.

The syntax of a try-catch block is as follows.

The code within the try block is executed. If there’s no error, the catch block is never executed. But, if there’s an error, the control passes to the catch block.

The catch block also contains a parameter that can be used to further extract error information. The code written in the catch block is executed. 

Developers can use the information provided by the caught exception to log the error by displaying a user-friendly message as well.

What is a JavaScript throw Statement?

The try-catch blocks are used to catch the already-known errors. But, the throw statement is a little different.

Throw statements are used to throw a custom error in JavaScript. When it is executed, the program flow is immediately interrupted, and the control is transferred to the nearest catch block.

One can define one’s errors according to particular conditions. The syntax is as follows:

where  error_expression represents the error that is being thrown by the function. It can be a JavaScript string, number, boolean, or even an object. 

Let us understand with an example example:

The code shows an error and doesn’t execute. The error occurs because the program is written to throw an error whenever the age is less than 18. But the program is not good enough to catch that error. This error has to be caught. Let us modify the code using the try-catch block using a try-catch block.

So, in the below example, we make use of the try-catch block wherever there’s a certainty of an error occurring:

Now, the code has been executed and gives us the right output. The second age, age2 is less than 18 when passed through the function throws an error that is caught by the catch block. The error message from the throw statement is hence printed.

Let us look into another example:

So, in this example, We have made a function to check the marks. If the marks are less than 50, an error is given which is caught by the catch block.  

There is one important thing to observe. When marks greater than 90 are passed through the function, the else statement of the function executes, and the line below the else statement executes because it’s part of the function. But, when marks less than 50 are passed and throw is executed, no statement below the throw statement will execute. Hence, we have got the desired output.

The throw keyword does a similar thing in Java as well. Using it, a developer can communicate that something unexpected has happened, preventing the program from continuing with its normal execution.

Effective error handling is a critical aspect of JavaScript programming. In this article, we studied the error-handling methods of try-catch and throw statements. By using them for error handling, one can create robust and reliable applications that handle errors.

Komal Bhatia

I am a dedicated Computer Science student with a strong track record of academic excellence. I am a highly motivated individual with a passion for providing creative and innovative solutions to complex problems. I possess skills in the domain of C++ and web development and am always eager to contribute to the field of Software Development.

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• try...catch

The try...catch statement marks a block of statements to try and specifies a response should an exception be thrown.

The statements to be executed.

Statement that is executed if an exception is thrown in the try -block.

An optional identifier to hold an exception object for the associated catch -block.

Statements that are executed after the try statement completes. These statements execute regardless of whether an exception was thrown or caught.

Description

The try statement consists of a try -block, which contains one or more statements. {} must always be used, even for single statements. At least one catch -block, or a finally -block, must be present. This gives us three forms for the try statement:

• try...finally
• try...catch...finally

A catch -block contains statements that specify what to do if an exception is thrown in the try -block. If any statement within the try -block (or in a function called from within the try -block) throws an exception, control is immediately shifted to the catch -block. If no exception is thrown in the try -block, the catch -block is skipped.

The finally -block will always execute after the try -block and catch -block(s) have finished executing. It always executes, regardless of whether an exception was thrown or caught.

You can nest one or more try statements. If an inner try statement does not have a catch -block, the enclosing try statement's catch -block is used instead.

You can also use the try statement to handle JavaScript exceptions. See the JavaScript Guide for more information on JavaScript exceptions.

Unconditional catch-block

When a catch -block is used, the catch -block is executed when any exception is thrown from within the try -block. For example, when the exception occurs in the following code, control transfers to the catch -block.

The catch -block specifies an identifier ( e in the example above) that holds the value of the exception; this value is only available in the scope of the catch -block.

Conditional catch-blocks

You can create "Conditional catch -blocks" by combining try...catch blocks with if...else if...else structures, like this:

A common use case for this is to only catch (and silence) a small subset of expected errors, and then re-throw the error in other cases:

The exception identifier

When an exception is thrown in the try -block, exception_var (i.e., the e in catch (e) ) holds the exception value. You can use this identifier to get information about the exception that was thrown. This identifier is only available in the catch -block's scope . If you don't need the exception value, it could be omitted.

The finally-block

The finally -block contains statements to execute after the try -block and catch -block(s) execute, but before the statements following the try...catch...finally -block. Note that the finally -block executes regardless of whether an exception is thrown. Also, if an exception is thrown, the statements in the finally -block execute even if no catch -block handles the exception.

The following example shows one use case for the finally -block. The code opens a file and then executes statements that use the file; the finally -block makes sure the file always closes after it is used even if an exception was thrown.

Nested try-blocks

First, let's see what happens with this:

Now, if we already caught the exception in the inner try -block by adding a catch -block

And now, let's rethrow the error.

Any given exception will be caught only once by the nearest enclosing catch -block unless it is rethrown. Of course, any new exceptions raised in the "inner" block (because the code in catch -block may do something that throws), will be caught by the "outer" block.

Returning from a finally-block

If the finally -block returns a value, this value becomes the return value of the entire try-catch-finally statement, regardless of any return statements in the try and catch -blocks. This includes exceptions thrown inside of the catch -block:

The outer "oops" is not thrown because of the return in the finally -block. The same would apply to any value returned from the catch -block.

Specifications

Browser compatibility.

© 2005–2021 MDN contributors. Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/try...catch

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Debate over tight end value hovers over Brock Bowers’ draft prospects

FILE - Georgia tight end Brock Bowers responds to questions during NCAA college football Southeastern Conference Media Days, Tuesday, July 18, 2023, in Nashville, Tenn. Bowers is widely considered one of the top 10 players in next week’s NFL draft but because he plays a lower-premium position of tight end, there is much more uncertainty about how high he will get drafted.(AP Photo/George Walker IV, File)

FILE - Georgia tight end Brock Bowers (19) leaps over TCU safety Millard Bradford (28) during the second half of the national championship NCAA College Football Playoff game, Monday, Jan. 9, 2023, in Inglewood, Calif. In three seasons at Georgia, Bowers caught 175 passes for 2,538 yards in 40 games and scored 31 total touchdowns with five of them coming as a runner as the Bulldogs did whatever they could to get the ball in his hands. (AP Photo/Ashley Landis, File)

FILE - Georgia tight end Brock Bowers (19) catches a pass as Auburn safety Donovan Kaufman defends during the second half of an NCAA football game, Saturday, Sept. 30, 2023, in Auburn, Ala. As he showed in three seasons at Georgia, Bowers is a dynamic receiver with the ability to create separation, make contested catches and create big plays after the catch, along with being a more than capable blocker.(AP Photo/ Butch Dill, File)

FILE - Georgia tight end Brock Bowers (19) makes a touchdown catch against TCU safety Abraham Camara (14) during the second half of the national championship NCAA College Football Playoff game, Monday, Jan. 9, 2023, in Inglewood, Calif. As he showed in three seasons at Georgia, Bowers is a dynamic receiver with the ability to create separation, make contested catches and create big plays after the catch, along with being a more than capable blocker.(AP Photo/Mark J. Terrill, File)

FILE - Atlanta Falcons tight end Kyle Pitts (8) runs down the field against the New York Jets during the first quarter of an NFL football game, Sunday, Dec. 3, 2023, in East Rutherford, N.J. Pitts has yet to make a big impact in his first three seasons after being drafted fourth overall in 2021 by Atlanta. (AP Photo/Seth Wenig, File)

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Evaluating Brock Bowers the player is a fairly straightforward assignment.

As he showed in three seasons at Georgia, Bowers is a dynamic receiver with the ability to create separation, make contested catches and create big plays after the catch, along with being a more than capable blocker.

Bowers is widely considered one of the top 10 players in next week’s NFL draft but because he plays the lower-premium position of tight end, there is much more uncertainty about how high he will get drafted.

While there could be teams in the top 10 willing to make the rare investment in a tight end, there is also the distinct possibility Bowers slips in the first round as teams prioritize high-value positions like quarterback, wide receiver, pass rusher, tackle or cornerback. Bowers is currently favored to be drafted outside the top 10, according to BetMGM Sportsbook .

“He is tough. He is easy to grade. I mean, when you watch him, he’s super easy to grade. He is one of the best 10 players in the draft,” said NFL Network draft analyst Daniel Jeremiah, who compared Bowers to All-Pro George Kittle and praised his tenacity, speed, ability to create separation and run after the catch.

“The challenge is then figuring out where does he go in the draft, and I think when you look around the league and you see most of these top tight ends that have come on day two or even beyond that, teams are now saying, ‘OK, we can find that other tight end. Maybe we don’t get the top guy, but we can get a really, really good player who might end up being the top guy without having to pay that premium.”

Of the top 15 tight ends in receptions last season, only four were first-round picks with only Buffalo rookie Dalton Kincaid and Cleveland’s David Njoku doing it for the team that drafted them. Five others were picked on the second day of the draft, with six more going the final day.

In all, 12 tight ends have been taken in the first round of the past 15 drafts with only one of those players — Atlanta’s Kyle Pitts — generating even one 1,000-yard receiving season. Njoku is the only one of seven first-round tight ends from 2014-20 to get a second contract with his team. The last draft with a first-round tight end who became a first-team All-Pro was 2003 with Dallas Clark of the Indianapolis Colts.

The question is can Bowers be the outlier in a class that has no other tight ends projected to go in the first round of the draft.

In three seasons at Georgia, he caught 175 passes for 2,538 yards in 40 games and scored 31 total touchdowns — five of them coming as a runner as the Bulldogs did whatever they could to get the ball in his hands.

“He’s not a tight end. He’s a multiple option player,” said ESPN draft analyst Mel Kiper Jr., who ranked Bowers as the seventh-best player in the draft. “You can put him in the backfield, slot, wing, outside, any which way, fullback, H-back. You can do anything you want with Brock Bowers. So he is not a tight end, he’s just an offensive weapon.”

Teams have been reticent about using high picks on tight ends, with just three going in the top 10 of the past 15 drafts — fewer than every position in that span outside of safeties, centers and specialists. Only nine other tight ends went in the first round since 2009, with the 12 total first-round picks ranking ahead of centers (10) and specialists.

With the rookie wage scale slotting salaries by pick regardless of position, it is more difficult for teams to generate surplus value by taking a tight end. Only two of the 126 players making an average of more than $15 million a year, according to Spotrac , are tight ends: Darren Waller and T.J. Hockenson.

Many of the top tight ends in the NFL have been picked on days two and three of the draft, including 2013 third-rounder Travis Kelce, 2017 fifth-rounder Kittle and even Sam LaPorta, who lasted until the second round last year for Detroit.

The payoff has been mostly underwhelming for the first-rounders, with none of those players earning first- or second-team All-Pro honors in their careers, including the three top 10 picks.

Eric Ebron lasted only four seasons in Detroit after being picked in 2014 with the 10th selection, 2019 No. 8 pick Hockenson elevated his game after being traded by the Lions to Minnesota midway through his fourth, and Pitts has yet to make a big impact in his first three seasons after being drafted fourth overall in 2021 by Atlanta.

Pitts’ lack of production the past two years with 81 catches for 1,023 yards could provide a cautionary tale for teams interested in using a high pick on Bowers.

“Kyle Pitts is as talented as any tight end that I have ever evaluated,” Jeremiah said. “You’re still dependent on the position of the quarterback, so depending on what you can get out of him. That coupled with the money difference, the savings you get by taking a premiere position in the top 10, it’s tough to place him to know how high he is going to go.”

AP NFL: https://apnews.com/hub/nfl