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Exception Handling in Node.js

Last Updated : 11 Apr, 2023
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Exception handling refers to the mechanism by which the exceptions occurring in a code while an application is running is handled. Node.js supports several mechanisms for propagating and handling errors. 
There are different methods that can be used for exception handling in Node.js: 

  • Exception handling in synchronous code: If an error occurs in a synchronous code, return the error. 

Example: 

javascript 
// Write Javascript code here
// Define divider as a synchronous function
let divideSync = function (x, y) {
    // if error condition?
    if (y === 0) {
        // "throw" the error safely by returning it
        return new Error("Can't divide by zero")
    }
    else {
        // no error occurred, continue on
        return x / y
    }
}

// Divide 9/3
let result = divideSync(9, 3)
// did an error occur?
if (result instanceof Error) {
    // handle the error safely
    console.log("9/3=err", result)
}
else {
    // no error occurred, continue on
    console.log("9/3=" + result)
}

// Divide 9/0
result = divideSync(9, 0)
// did an error occur?
if (result instanceof Error) {
    // handle the error safely
    console.log("9/0=err", result)
}
else {
    // no error occurred, continue on
    console.log("9/0=" + result)
}

Output: 

  • Exception handling in callback-based( asynchronous) code: In callback-based code, one of the arguments of the callback is erred. If an error happens err is the error, if an error doesn't happen then err is null. The err argument can be followed by any number of other arguments. 

Example: 

javascript 
// Write Javascript code here
let divide = function (x, y, next) {
    // if error condition?
    if (y === 0) {
        // "throw" the error safely by calling the completion callback
        // with the first argument being the error
        next(new Error("Can't divide by zero"))
    }
    else {
        // no error occurred, continue on
        next(null, x / y)
    }
}

divide(9, 3, function (err, result) {
    // did an error occur?
    if (err) {
        // handle the error safely
        console.log("9/3=err", err)
    }
    else {
        // no error occurred, continue on
        console.log("9/3=" + result)
    }
})

divide(9, 0, function (err, result) {
    // did an error occur?
    if (err) {
        // handle the error safely
        console.log("9/0=err", err)
    }
    else {
        // no error occurred, continue on
        console.log("9/0=" + result)
    }
})

Output: 
 

  • Exception handling in eventful code: In an eventful code, the error may happen anywhere. So instead of throwing the error, fire the error event instead. 

Example: 

javascript 
// Write Javascript code here
// Definite our Divider Event Emitter
const events = require("events")
let Divider = function () {
    events.EventEmitter.call(this)
}
require('util').inherits(Divider, events.EventEmitter)

// Add the divide function
Divider.prototype.divide = function (x, y) {
    // if error condition?
    if (y === 0) {
        // "throw" the error safely by emitting it
        var err = new Error("Can't divide by zero")
        this.emit("error", err)
    }
    else {
        // no error occurred, continue on
        this.emit("divided", x, y, x / y)
    }

    // Chain
    return this;
}

// Create our divider and listen for errors
let divider = new Divider()
divider.on('error', function (err) {
    // handle the error safely
    console.log(err)
})
divider.on('divided', function (x, y, result) {
    console.log(x + "/" + y + "=" + result)
})

// Divide
divider.divide(9, 3).divide(9, 0)

Output: 


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