Exception handling in JavaScript

Written by Deepak Gupta✏️

Errors are part of the programming journey. By producing errors, we actually learn how not to do something and how to do it better next time.

In JavaScript, when code statements are tightly coupled and one generates an error, it makes no sense to continue with the remaining code statements. Instead, we try to recover from the error as gracefully as we can. The JavaScript interpreter checks for exception handling code in case of such errors, and if there is no exception handler, the program returns whatever function caused the error.

This is repeated for each function on the call stack until an exception handler is found or the top-level function is reached, which causes the program to terminate with an error.

In general, exceptions are handled in two ways:

1. Throw an exception — If there is a problem that can’t be handled meaningfully where it occurs at runtime, it’s best to throw it

function openFile(fileName) {
    if (!exists(fileName)) {
        throw new Error('Could not find file '+fileName); // (1)
    }
    ...
}

1. Catch an exception —  Thrown exceptions are caught and handled at the place where they make more sense at runtime

try {
  openFile('../test.js');
} catch(e) {
// gracefully handled the thrown expection 
}

Let’s dive into these actions in more detail.

Throw an exception

If you’ve been using JavaScript for a long time, you may have seen something like ReferenceError: fs is not defined. This represents an exception that was thrown via a throw statement.

Syntax

throw «value»;
// Don't do this
if (somethingBadHappened) {
    throw 'Something bad happened';
}

There is no restriction on the type of data that can be thrown as an exception, but JavaScript has special built-in exception types. One of them is Error, as you saw in the previous example. These built-in exception types give us more details than just a message for an exception.

Error

The Error type is used to represent generic exceptions. This type of exception is most often used for implementing user-defined exceptions. It has two built-in properties to use.

1. message

This is what we pass as an argument to the Error constructor — e.g., new Error('This is the message'). You can access the message through the message property.

const myError = new Error(‘Error is created’)
console.log(myError.message) // Error is created

2. stack

The stack property returns the history (call stack) of what files were responsible for causing the error. The stack also includes the message at the top and is followed by the actual stack, starting with the most recent/isolated point of the error to the most outward responsible file.

Error: Error is created
at Object. (/Users/deepak/Documents/error-handling/src/index.js:1:79)
at Module.compile (internal/modules/cjs/loader.js:689:30)
at Object.Module.extensions..js (internal/modules/cjs/loader.js:700:10)
at Module.load (internal/modules/cjs/loader.js:599:32)
at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
at Function.Module._load (internal/modules/cjs/loader.js:530:3)
at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
at startup (internal/bootstrap/node.js:266:19)
at bootstrapNodeJSCore (internal/bootstrap/node.js:596:3)

Note: new Error('...') does not do anything until it’s thrown — i.e., throw new Error('error msg')   will create an instance of an Error in JavaScript and stop the execution of your script unless you do something with the Error, such as catch it.

Catch an exception

Now that we know what exceptions are and how to throw them, let’s discuss how to stop them from crashing our programs by catching them.

try-catch-finally

This is the simplest way to handle the exceptions. Let’s look at the syntax.

try {
    // Code to run
  } catch (e) {
    // Code to run if an exception occurs
  }
  [ // optional
    finally {
      // Code that is always executed regardless of 
      // an exception occurring
    }
  ]

In the try clause, we add code that could potentially generate exceptions. If an exception occurs, the catch clause is executed.

Sometimes it is necessary to execute code whether or not it generates an exception. Then we can use the optional block finally.

The finally block will execute even if the try or catch clause executes a return statement. For example, the following function returns false because the finally clause is the last thing to execute.

function foo() {
  try {
    return true;
  } finally {
    return false;
  }
}

We use try-catch in places where we can’t check the correctness of code beforehand.

const user = '{"name": "Deepak gupta", "age": 27}';
try {
  // Code to run
  JSON.parse(params)
  // In case of error, the rest of code will never run
  console.log(params)
} catch (err) {
  // Code to run in case of exception
  console.log(err.message)
}

As shown above, it’s impossible to check the JSON.parse to have the stringify object or a string before the execution of the code.

Note: You can catch programmer-generated and runtime exceptions, but you can’t catch JavaScript syntax errors.

try-catch-finally can only catch synchronous errors. If we try to use it with asynchronous code, it’s possible that try-catch-finally will have already been executed before the asynchronous code finishes its execution.

How to handle exceptions in an asynchronous code block

JavaScript provides a few ways to handle exceptions in an asynchronous code block.

Callback functions

With callback functions (not recommended) ,  we usually receive two parameters that look something like this:

asyncfunction(code, (err, result) => {
    if(err) return console.error(err);
    console.log(result);
})

We can see that there are two arguments: err and result. If there is an error, the err parameter will be equal to that error, and we can throw the error to do exception handling.

It is important to either return something in the if(err) block or wrap your other instruction in an else block. Otherwise, you might get another error — e.g., result might be undefined when you try to access result.data.

Promises

With promises — then or catch — we can process errors by passing an error handler to the then method or using a catch clause.

promise.then(onFulfilled, onRejected)

It’s also possible to add an error handler with .catch(onRejected) instead of .then(null, onRejected), which works the same way.

Let’s look at a .catch example of promise rejection.

Promise.resolve('1')
  .then(res => {
      console.log(res) // 1
      throw new Error('something went wrong'); // exception thrown 
})
.then(res => {
      console.log(res) // will not get executed
})
.catch(err => { 
      console.error(err) // exception catched and handled
});

async and await with try-catch

With async/await and try-catch-finally, handling exceptions is a breeze.

async function() {
    try {
        await someFuncThatThrowsAnError()
    } catch (err) {
        console.error(err) 
    }
})

How to handle uncaught exceptions

Now that we have a good understanding of how to perform exception handling in synchronous and asynchronous code blocks, let’s answer the last burning question of this article : how do we handle uncaught exceptions?

In the browser

The method window.onerror() causes the error event to be fired on the window object whenever an error occurs during runtime. We can use this method to handle the uncaught exception.

Another utility mode for onerror() is using it to display a message in case there is an error when loading images in your site.

<img src="testt.jpg" onerror="alert('An error occurred loading yor photo.')" />

On a Node.js server

The process object derived from the EventEmitter module can be subscribed to the event uncaughtException.

process.on("uncaughtException", () => {})`

We can pass a callback to handle the exception. If we try to catch this uncaught exception, the process won’t terminate, so we have to do it manually.

The uncaughtException only works with synchronous code. For asynchronous code, there is another event called unhandledRejection.

process.on("unhandledRejection", () => {})

Never try to implement a catch-all handler for the base Error type. This will obfuscate whatever happened and compromise the maintainability and extensibility of your code.

Key takeaways

Let’s review some of the main points we discussed in this article.

• The throw statement is used to generate user-defined exceptions. During runtime, when a throw statement is encountered, execution of the current function will stop and control will be passed to the first catch clause in the call stack. If there is no catch clause, the program will terminate
• JavaScript has some built-in exception types, most notably Error, which returns the error stack and message
• The try clause will contain code that could potentially generate an exception
• The catch clause will be executed when exceptions occur
• For asynchronous code, it’s best to use async-await with try-catch
• An unhandled exception can be caught, which can prevent the app from crashing

When done properly throughout, exception handling can help you improve the maintainability, extensibility, and readability of your code.

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