Most of the constructs we've looked at so far are used for synchronous code, where statements are evaluated sequentially, with each line completing before the next one begins:
const someValue = getSomeValue();
doSomethingWithTheValue(someValue);
Code like this is straightforward. We intuitively understand that these two lines of code will run one after the other. There is also an assumption that neither of these lines will take very long to execute, probably taking no more than a few micro- or milliseconds.
But what happens if we wish to bind to a user Event or fetch some remote data? These are things that take time and will only complete when some future Event occurs. In a less kind universe, there would be no way to deal with such scenarios other than simply waiting for them to complete and then continuing the execution of our program:
fetchSomeData...
