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

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.

In a perfect world, where there's no bugs (because code just fixes itself magically), and requirements are frozen from the day one, it may be possible to live with huge omnipotent functions.

But in this world it turns to be just too expensive - and not only in terms of 'man-month'. Nicholas Zakas wrote a brilliant article describing most of the challenges software developers face these days.

The transition may seem somewhat artificial, but my point is that 'one function - one task' approach is much more maintainable and flexible - in other words, it's what makes BOTH developers and customers happy, in the end.

It doesn't mean, though, that you'd not strive to use as few function calls as possible: just remember that it's not a top priority.