10

I'm currently learning about callbacks in Node and JavaScript in general and am getting confused by the following:

var request = require('request');

request('http://www.google.com', function (error, response, body) {
 if (!error && response.statusCode == 200) {
console.log(body) // Show the HTML for the Google homepage. 
  }
})

My question is this: How does the request function know what each parameter/argument is in the callback? Because I could effectively call the function callback with two parameters and skip out the error? How would the function know that the first parameter passed was the response and not the error, for instance?

Does it do checks on the types of each at runtime or? Thanks.

Improve this question

4 Answers 4

Reset to default
10

The programmer who wrote the request API decides the order of the parameters sent to the callback. There is no type checking at runtime. If you were to supply only two arguments in your callback signature, e.g.

function(response, body) {}

you would still be receiving the error object and the response -- only in this case, the variable named "response" would hold the error object, and the variable named "body" would hold the response. The input you would be skipping would be the third one you didn't supply, the body. The names of your parameters make no difference, it is their position in the function signature that determines what value they will be assigned.

By convention, in javascript callbacks, the first parameter of the callback normally represents the error object. It is not always written this way, but it is a best practice.

Improve this answer
Sign up to request clarification or add additional context in comments.

1 Comment

That makes sense to me! Thank you for clearing it up for me!
5

Possibile implementation of this function could be:

function request (url, callback) {
  // doing something

  callback (error, response, body);
}

The function will set the value of error, response and body parameters. I don't know how it's is implemented, but suppose that you pass a number for the url. A possibile implementation could be:

function request (url, callback) {
  if (typeof url === 'number') {
    // Suppose that error is an instance of Error
    callback (new Error ('Url must be a string'), null, null);
  }
  // other stuff...
}
Improve this answer

Comments

1

When you call the request() function, and provide a function as an argument, it calls that function internally (inside the module) and passes in it's own data as the arguments (which you then use to do various things). The data it passes in is data it generated itself, it's not data you're creating.

That is a callback.

There is a quetsion I answered a little while ago that goes more indepth as to why these modules are structured this way, see: npm's guidelines on callback errors

Improve this answer

1 Comment

Thanks, Jamen. I'll take a look now.
0

You are using a specific module my friend and callbacks are controlled in this case directly from it.

How the callback works for 'Request' module is matter of research. Check their documentation.

Improve this answer

2 Comments

What if I wanted to implement a function that accepted a callback with numerous arguments? Could you give me an example of how I could do that and match each parameter to its correct method or property? Thanks!
Callbacks are associated with part to which request is being made. Keeping your request in mind, let's say I am developing a module like Request then I can make arrangements for as many callbacks I want, what work I would assign to them is imaginary for now xD

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.