Javascript function as a parameter to another function?

Question

I\'m learning lots of javascript these days, and one of the things I\'m not quite understanding is passing functions as parameters to other functions. I get the concept

Answer 1

This probably won't be of much practical use to you as a web programmer, but there is another class of uses for functions as first-class objects that hasn't come up yet. In most functional languages, like Scheme and Haskell, passing functions around as arguments is, along with recursion, the meat-and-potatoes of programming, rather than something with an occasional use. Higher-order functions (functions that operate on functions) like map and fold enable extremely powerful, expressive, and readable idioms that are not as readily available in imperative languages.

Map is a function that takes a list of data and a function and returns a list created by applying that function to each element of the list in turn. So if I wanted to update the positions of all the bouncing balls in my bouncing ball simulator, instead of

for(ball : ball_list) {
   ball.update();
   ball.display();       
}

I would instead write (in Scheme)

(display (map update ball-list))

or in Python, which offers a few higher-order functions and a more familiar syntax,

display( map(update, ball-list) )

Fold takes a two-place function, a default value, and a list, and applies the function to the default and the first element, then to the result of that and the second element, and so on, finally returning the last value returned. So if my server is sending in batches of account transactions, instead of writing

for(transaction t : batch) {
    account_balance += t;
}

I would write

(fold + (current-account-balance) batch))

These are just the simplest uses of the most common HOFs.