Functions can indeed return functions. Take this function, for example:
function func(text) {
alert(text);
return func;
}
Obviously the return value of any invocation of func
will be, again, func
, so you can use it like this:
func("hello")("world");
…and you'll get two alerts: first "hello", and then "world".
Next, there's something called a named function expression. You might have seen anonymous function expressions before:
doSomething(thing, function(err) {
// operation completed or something
});
That, of course, is great for simple things, but sometimes you want the function to have a name so it can refer to itself. As Kolink mentioned, if you just want to recurse, there's arguments.callee
, which refers to the function currently executing, but there is another way: you can give the function a name visible only within the function while still having it be a function expression:
doSomething(thing, function myself(err) {
// ^^^^^^
// now I can refer to myself as myself!
});
An underscore is a valid identifier, so they're basically just combining these techniques in a way that may be difficult to understand.