Exploring the concept of callbacks when using the http.get function in node

Question

Exploring the concept of callbacks when using the http.get function in node

As a new node programmer, I am struggling to grasp how to retrieve the contents of an HTTP request from a function due to the asynchronous nature of node. Below is a simplified version of my program:

//#1
function getWebsiteData(url) {
    var body;
    http.get(url, function (res) {
        res.on('data', function (chunk) {
            body += chunk;
            //***At this point, I need 'body' to be returned from this function***
        });
    });
}


//#2
var dataA = getWebsiteData('website1.com');
var dataB = getWebsiteData('website2.com');

//#3
console.log(dataA + dataB);

My goal is simple: 1: create a function that retrieves data from a site (specifically JSON and XML), 2: call that function from any part of my program, 3: manipulate the data returned by the function.

The placement and calling of callback functions in node.js are causing me quite a headache. Despite studying numerous examples of http.get and callbacks, I have not been able to find one where they mesh together like in my sample code. After a full day of unsuccessful attempts, I am hopeful that seeing how to implement this in my example will finally make it click in my mind (fingers crossed).

javascript asynchronous callback

Answer 1

Answer №1

Here is the fundamental pattern to follow for using plain callback style:

function fetchWebsiteData(url, callback) {
    let content = "";
    http.get(url, function (response) {
        response.on('data', function (chunk) {
            content += chunk.toString();
            // Returning a value from this function won't work
            // The return value will be consumed by the function
            // that calls this function, which is internal Node.js code
        });
        response.on("end", function() {
          callback(null, content)
        })
        response.on("error" function(error) {
          callback(error)
        })
    });
}

fetchWebsiteData("a", function(error, dataA) {
  if (error) throw "Error fetching website A."
  fetchWebsiteData("b", function(error, dataB) {
    if (error) throw "Error fetching website B."
    console.log(dataA)
    console.log(dataB)
  })
})

You might also consider utilizing flow-control libraries like Async (or my promise-based Faithful), or general-purpose promise libraries like RSVP.js. I suggest getting familiar with callbacks basics first.

For simple http requests, it's more convenient to use the request module. No need to attach event listeners. Just a function similar to the fetchWebsiteData function (which has its own issues!).

To grasp async programming, try reading and writing files with fs.readFile and fs.writeFile. Attempt something like writing contents of file A to file B. These are simpler functions, but you must manage callback flow correctly. Node's http module is comparatively complex. You can opt for the request module as an alternative.

Answer 2

Here is the fundamental pattern to follow for using plain callback style:

function fetchWebsiteData(url, callback) {
    let content = "";
    http.get(url, function (response) {
        response.on('data', function (chunk) {
            content += chunk.toString();
            // Returning a value from this function won't work
            // The return value will be consumed by the function
            // that calls this function, which is internal Node.js code
        });
        response.on("end", function() {
          callback(null, content)
        })
        response.on("error" function(error) {
          callback(error)
        })
    });
}

fetchWebsiteData("a", function(error, dataA) {
  if (error) throw "Error fetching website A."
  fetchWebsiteData("b", function(error, dataB) {
    if (error) throw "Error fetching website B."
    console.log(dataA)
    console.log(dataB)
  })
})

You might also consider utilizing flow-control libraries like Async (or my promise-based Faithful), or general-purpose promise libraries like RSVP.js. I suggest getting familiar with callbacks basics first.

For simple http requests, it's more convenient to use the request module. No need to attach event listeners. Just a function similar to the fetchWebsiteData function (which has its own issues!).

To grasp async programming, try reading and writing files with fs.readFile and fs.writeFile. Attempt something like writing contents of file A to file B. These are simpler functions, but you must manage callback flow correctly. Node's http module is comparatively complex. You can opt for the request module as an alternative.

Answer 3

Answer №2

Before the data is returned, you're calling console.log(). To fix this issue, consider bringing the body variable to a higher scope and appending to that instead.

let body;

function fetchWebsiteData(url) {
    http.get(url, function (res) {

        res.on('data', function (chunk) {
            body += chunk;
            //***At this point, I require 'body' to be returned outside of this function***
            displayBody(body);
        });
    });
}
let siteOne = fetchWebsiteData('website1.com');
let siteTwo = fetchWebsiteData('website2.com');

function displayBody(value){
    console.log(value);
}

Answer 4

Before the data is returned, you're calling console.log(). To fix this issue, consider bringing the body variable to a higher scope and appending to that instead.

let body;

function fetchWebsiteData(url) {
    http.get(url, function (res) {

        res.on('data', function (chunk) {
            body += chunk;
            //***At this point, I require 'body' to be returned outside of this function***
            displayBody(body);
        });
    });
}
let siteOne = fetchWebsiteData('website1.com');
let siteTwo = fetchWebsiteData('website2.com');

function displayBody(value){
    console.log(value);
}

Exploring the concept of callbacks when using the http.get function in node

Answer №1

Answer №2

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