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Exception Handling in Node

  • Last Updated : 15 Nov, 2019

The 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.
This are the different methods which 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:




    // Write Javascript code here
    // Define divider as a syncrhonous function
    var 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
    var 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, the one of the argument of the callback is err. If an error happens err is the error, if an error doesn’t happen then err is null. The err argument can be followed any number of other arguments.
    Example:




    // Write Javascript code here
    var 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:




    // Write Javascript code here
    // Definite our Divider Event Emitter
    var events = require("events")
    var 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
    var 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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