timed.js

// The following function is a combinator.  It takes a function as an
// argument, and returns a function.
function timed(subject) {
  return function(...args) {
    const start = Date.now();
    const result = subject.apply(null, args);
    const end = Date.now();

    console.log("timed function took: " + (end - start) + "ms");
    return result;
  };
}

function sillyFunction() {
  for (let i=0; i<10000000; ++i) {
    // Taking the CPU for a spin.
  }
}

const timedSillyFunction = timed(sillyFunction);
timedSillyFunction();

const timedAdder = timed(n => n + 1);
const r1 = timedAdder(1); // 2
const r2 = timedAdder(19); // 20

console.assert(r1 === 2);
console.assert(r2 === 20);