Node.js Exception handling Handling Exception In Node.Js
Example
Node.js has 3 basic ways to handle exceptions/errors:
- try-catch block
- error as the first argument to a
callback emitan 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
});
