When you change the number in the input box, the code you see will be executed with that parameter. The call tree will be shown below.

The call tree is built using the data provided by blunt-instrument about what actually occurred during execution of the code; no logging or display code was manually added to the function. View this page's source to see how it works, or visit the documentation.

function fib(n) {
  if (n < 2) {
    return n;
  }
  return fib(n - 2) + fib(n - 1);
}