Node.js: Juggling Async (learnyounode). Recursion in callback functions not working as expected

Question

So I was solving this learnyounode exercise called Juggling Async , which basically gives me 3 URLs and asks me to output the URL content, in the order of the URLs input.

I came up with a recursive solution, in which a callback calls another callback while maintaining an exit counter num . However, when I run this, I keep getting blank lines on the console, no matter what the input.

This is the short code:

var url = ["", process.argv[2], process.argv[3], process.argv[4]];
var http = require('http');
var str = ["", "", "", ""];

var cnt = 1;

function dos(num, callback)
{
    if(num == 4)
        return;
    http.get(url[num], function (response1){
    //  console.log(num);
        response1.setEncoding('utf8');
        response1.on('data', function(data1) {
            str[num] = data1;
            callback(num + 1, dos);

        })
    });
}

dos(1, dos);
console.log(str[1]);
console.log(str[2]);
console.log(str[3]);

Now, this is my first time pulling off recursion with callback functions. I learned callback functions just two days ago, which might mean that there is something glaringly wrong here which I cannot see. Please help me see. Thank you.

Answer 1

Look at the execution order.

1. You define the dos function
2. You invoke the dos function
3. You invoke the console.log

The problem with your code is asynchronous nature of the http.get method.

In the definition of dos whenever http.get is invoked, a callback is attached and after that control flow exists the function scope. Control flow is not stopping and waiting for the get call to complete.

It exits the function scope and moves onto invoking console.log so you get blank strings because that's what your array contains at that moment.

For you to achieve your expected result.

In the function call instead of assigning the value of data1 to str[num] . Do a simple console.log(data1) in place.

If you still prefer the outside console.log placement in your code. You read about async\\await .

dos invokation must return a Promise .