gistfile1.js

var isNumber = (function(){
  var toString = Object.prototype.toString;
  return function(o) {
    return toString.call(o) === '[object Number]';
  }
})();

function isNumber2(o) {
  try {
    return typeof (o && o.valueOf()) == 'number';
  } catch(e) {}
  return false;
}

console.time('class');
for (var i=0; i<10000; i++) { isNumber(10) }
console.timeEnd('class');

console.time('valueof');
for (var i=0; i<10000; i++) { isNumber2(10) }
console.timeEnd('valueof');

// class: 41ms
// valueof: 18ms