"Function calls are expensive" vs. "Keep functions small"

Question

On the one hand, I read or hear that "function calls are expensive" and that they impact efficiency (for example, on Nicholas Zakas' Google tech talk).

On the other hand, however, it seems accepted that functions/methods are best kept short and should only really perform one task, as generally accepted in here.

Am I missing something here, or don't these two pieces of advice run contrary to one another? Is there some rule-of-thumb that allows one to maintain a Zen-like balance?

Answer 1

The general rule applying to all languages is: keep functions (methods, procedures) as small as possible. When you add proper naming, you get very maintainable and readable code where you can easily focus on general picture and drill down to interesting details. With one huge method you are always looking at the details and the big picture is hidden.

This rule applies specifically to clever languages and compiler that can do fancy optimizations like inlining or discovering which methods aren't really virtual so double dispatch isn't needed.

Back to JavaScript - this is heavily dependant on JavaScript engine. In some cases I would expect decent engine to inline function, avoiding the cost of execution, especially in tight loops. However, unless you have a performance problem, prefer smaller functions. Readability is much more important.