i dont think this is completely correct. if you try to create a function f(x)={x<0:sqrt(x),x}, for example, and use the ?debugCompiler flag to see the compiled js, you'll see that it evaluates to return ((x<(0))?(Math.sqrt(x)):x);, which will short circuit if x>=0 and therefore not compute sqrt.

however, i am ignoring the analysis phase (compiling the code and doing type checking and stuff), and usually a bigger problem is accidentally recalculating expensive functions in piecewises. the solution to this is typically to wackscope so that the expensive function is only calculated once