What is callback hell and how and why does RX solve it

Question

Can someone provide a precise description and a straightforward example that clarifies what a "callback hell" is for someone who is unfamiliar with JavaScript and node.js?

When (and under what conditions) does the "callback hell issue" manifest itself?

How does it happen?

Do asynchronous calculations and "callback hell" always go hand in hand?

Additionally, can a single-threaded programme experience "callback hell"?

Erik Meijer stated in one of his lectures for the Coursera Reactive course that RX eliminates the issue of "callback hell."

On the Coursera forum, I questioned what a "callback hell" was, but I didn't get a clear response.

Could you demonstrate how RX resolves the "callback hell problem" on that basic example after defining "callback hell" with a simple

example?

Answer 1

The issue with Javascript is that you must place "the rest of it" within a callback in order to "freeze" a calculation and have the "rest of it" execute later (asynchronously).

Say, for instance, that I wish to execute the following code:

x = getData();
y = getMoreData(x);
z = getMoreData(y);
...

What would happen if I wanted to make the getData methods asynchronous so that I could execute other code while I waited for their data to return? The only solution in Javascript would be to completely rebuild everything that interacts with an async computation in continuation passing style:

getData(function(x){
    getMoreData(x, function(y){
        getMoreData(y, function(z){ 
            ...
        });
    });
});