measure function execution

perf.js

function perf(self, func) {
  // everything is argument except for 'self' and 'func'
  var args = Array.prototype.slice.call(arguments, 2, arguments.length);

  var t1 = performance.now();
  func.apply(self, args);
  var t2 = performance.now();
  var t = t2 - t1;

  return t.toFixed(2) + 'ms' + ', ' + (t / 1000).toFixed(2) + 's';
}

// usage
// perf(null, f, 1, 2, 3);
// perf(obj, obj.doSomething, 1, 2, 3);