CODEWITHSUNDEEP
javascriptnode.js
RJ Stank
RJ Stank

Reputation: 31

Node.js Callback Function Error Parameter Explanation?

I have a basic understanding of javascript and have been learning how asynchronous functions work in node.js. I've been very confused by callback functions with the parameter error. For example, here's some code:

contact.saveContacts = function(contactArray, done) {
       var jsonfile = require('jsonfile')
       jsonfile.writeFile('data.json', contactArray, done)
}

Contact.saveContacts(contacts, function(err) {
       console.log('success')
}

My question is, why does the callback function contain the parameter error? I'm confused to why it's there because it seems as if it serves no purpose in the function it calls.

Upvotes: 2

Views: 1045

Answers (4)

pmccloghrylaing
pmccloghrylaing

Reputation: 1129

Typically synchronous functions either return successfully, possibly with a value, or throw an exception if there is a problem. The calling code can choose what to do if an exception is thrown, by either catching and inspecting the error or by letting it fall through to other code that may handle the error.

Asynchronous callback functions are called after the calling code has already executed. This means there's no opportunity to catch thrown exceptions. So instead of throwing, errors are passed through to the callback function so the calling code can handle both success and error states.

Upvotes: 1

danillouz
danillouz

Reputation: 6371

This is a pattern called an error first callback and is used a lot in javascript.

See this article for reference.

Upvotes: 3

amulya349
amulya349

Reputation: 1238

The error parameter has no usage if everything goes fine. But, its too useful when there comes an error. Any type of error e.g. runtime error, of file has been deleted or anything, if happens, the details of the error will be present in the error parameter of the callback. So, its better to use that parameter as following:

Contact.saveContacts(contacts, function(err) {
    if(err){
        console.log(err);
    }
    else{
        console.log('success');
    }

}

In this way, you will get to know any error, if that happens with the function.

Upvotes: 0

nikoss
nikoss

Reputation: 3688

in case there is a problem with write operation such as permissions error object is invoked and the reason in this to prevent unexpected errors. Imagine we are giving a order to do computer starts doing it but on the way theres a block about permissions that computer cannot write into that dir in this case computer doesnt know what to do and our program crashes to prevent this inside callback we specify what to do in such cases for example if permission is denied and the reason is write permission prompt user for password and force writing or open a box to user that user must run this as user

Upvotes: 0

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