Exception handling

Handling Exception In Node.Js

Node.js has 3 basic ways to handle exceptions/errors:

try-catch block
error as the first argument to a callback
emit an error event using eventEmitter

try-catch is used to catch the exceptions thrown from the synchronous code execution. If the caller (or the caller’s caller, …) used try/catch, then they can catch the error. If none of the callers had try-catch than the program crashes.
If using try-catch on an async operation and exception was thrown from callback of async method than it will not get caught by try-catch. To catch an exception from async operation callback, it is preferred to use promises.
Example to understand it better

// ** Example - 1  **
function doSomeSynchronousOperation(req, res) {
    if(req.body.username === ''){
        throw new Error('User Name cannot be empty');
    }  
    return true;  
}

// calling the method above
try {
    // synchronous code   
    doSomeSynchronousOperation(req, res)    
catch(e) {
    //exception handled here   
    console.log(e.message);  
} 

// ** Example - 2 **
function doSomeAsynchronousOperation(req, res, cb) {
    // imitating async operation
    return setTimeout(function(){
        cb(null, []);
    },1000);
}
 
try {
    // asynchronous code   
    doSomeAsynchronousOperation(req, res, function(err, rs){
        throw new Error("async operation exception");
    })   
} catch(e) {
     // Exception will not get handled here
     console.log(e.message);  
}
// The exception is unhandled and hence will cause application to break

callbacks are mostly used in Node.js as callback delivers an event asynchronously. The user passes you a function (the callback), and you invoke it sometime later when the asynchronous operation completes.
The usual pattern is that the callback is invoked as a callback(err, result), where only one of err and result is non-null, depending on whether the operation succeeded or failed.

function doSomeAsynchronousOperation(req, res, callback) {
   setTimeout(function(){
        return callback(new Error('User Name cannot be empty'));    
   }, 1000);  
   return true;
}

doSomeAsynchronousOperation(req, res, function(err, result) {
   if (err) {
       //exception handled here 
       console.log(err.message);
   }
   
   //do some stuff with valid data
});

emit For more complicated cases, instead of using a callback, the function itself can return an EventEmitter object, and the caller would be expected to listen for error events on the emitter.

const EventEmitter = require('events');

function doSomeAsynchronousOperation(req, res) {
    let myEvent = new EventEmitter();

    // runs asynchronously
    setTimeout(function(){
        myEvent.emit('error', new Error('User Name cannot be empty'));
    }, 1000);

    return myEvent;
}

// Invoke the function
let event = doSomeAsynchronousOperation(req, res);

event.on('error', function(err) {
    console.log(err);
});

event.on('done', function(result) {
    console.log(result); // true
});

Unhanded Exception Management

Because Node.js runs on a single process uncaught exceptions are an issue to be aware of when developing applications.

Silently Handling Exceptions

Most of the people let node.js server(s) silently swallow up the errors.

Silently handling the exception

process.on(‘uncaughtException’, function (err) { console.log(err); });

This is bad, it will work but:

Root cause will remains unknown, as such will not contribute to resolution of what caused the Exception ( Error ).
In case of database connection ( pool ) gets closed for some reason this will result in constant propagation of errors, meaning that server will be running but it will not reconnect to db.

Returning to Initial state

In case of an ” uncaughtException ” it is good to restart the server and return it to its initial state, where we know it will work. Exception is logged, application is terminated but since it will be running in a container that will make sure that the server is running we will achieve restarting of the server ( returning to the initial working state ) .

Installing the forever ( or other CLI tool to make sure that node server runs continuously )

npm install forever -g
Starting the server in forever

forever start app.js

Reason why is it started and why we use forever is after the server is terminated forever process will start the server again.

Restarting the server

process.on(‘uncaughtException’, function (err) { console.log(err);
```
  // some logging mechanisam
  // ....        

  process.exit(1); // terminates process
```
});

On a side note there was a way also to handle exceptions with Clusters and Domains.

Domains are deprecated more information here.

Errors and Promises

Promises handle errors differently to synchronous or callback-driven code.

const p = new Promise(function (resolve, reject) {
    reject(new Error('Oops'));
});

// anything that is `reject`ed inside a promise will be available through catch
// while a promise is rejected, `.then` will not be called
p
    .then(() => {
        console.log("won't be called");
    })
    .catch(e => {
        console.log(e.message); // output: Oops
    })
    // once the error is caught, execution flow resumes
    .then(() => {
        console.log('hello!'); // output: hello!
    });

currently, errors thrown in a promise that are not caught results in the error being swallowed, which can make it difficult to track down the error. This can be solved using linting tools like eslint or by ensuring you always have a catch clause.

This behaviour is deprecated in node 8 in favour of terminating the node process.