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
});

Found a mistake? Have a question or improvement idea? Let me know.

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