Question

if I have a function defined earlier, do I need to include parenthesis when specifying that it should be used for a success callback?

what would be the difference if I did?

as in

function fish_food(){//do something}

$.ajax({
    url: '/',
    success: fish_food
});

or

$.ajax({
    url: '/',
    success: fish_food()
});

Answer 1

fish_food on its own (without parens) acts as a reference to the function object. It allows you to pass the reference to the function around to be invoked at some later date.

fish_food() (with parens) is a function invocation expression, which causes the function to be executed. The function code is evaluated and run with a value optionally being returned.

With the AJAX code you supplied (and all async JavaScript involving callbacks) you want to use the fish_food version (without parens). This passes the AJAX code a reference to your success function, to be executed asynchronously once the AJAX code has completed its round trip to the server and back.