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How do you handle errors in a NodeJS application?

An error is any problem given out by the program due to a number of factors such as logic, syntax, timeout, etc. An error in Node.js is any instance of the Error object. Common examples include built-in error classes, such as ReferenceError, RangeError, TypeError, URIError, EvalError, and SyntaxError.

User-defined errors can also be created by extending the base Error object, a built-in error class, or another custom error. In general, Node.js errors are divided into two distinct categories: operational errors and programmer errors.

1. Operational Errors:

Operational errors represent runtime problems. These errors are expected in the Node.js runtime and should be dealt with in a proper way. Here's a list of common operational errors:

  • failed to connect to server
  • failed to resolve hostname
  • invalid user input
  • request timeout
  • server returned a 500 response
  • socket hang-up
  • system is out of memory

2. Programmer Errors:

Programmer errors are what we call bugs. They represent issues in the code itself. Here's a common one for Node.js, when you try reading a property of an undefined object. It's a classic case of programmer error. Here are a few more:

  • called an asynchronous function without a callback
  • did not resolve a promise
  • did not catch a rejected promise
  • passed a string where an object was expected
  • passed an object where a string was expected
  • passed incorrect parameters in a function

Error handling​

In Node.js, errors can be handled using try-catch blocks or by listening for the 'error' event on an event emitter. It is also a good practice to use a global error-handling middleware function to catch unhandled errors.

Unhandled exceptions in Node.js can be caught at the Process level by attaching a handler for uncaughtException event.

process.on('uncaughtException', function(err) {
    console.log('Caught exception: ' + err);
});

Process is a global object that provides information about the current Node.js process. Process is a listener function that is always listening to events.

Few events are :

  1. exit
  2. disconnect
  3. unhandledException
  4. rejectionHandled