simple-check.js

function typeCheck(obj, type) {
  if (type === undefined || type === null) {
    return obj === type;
  }
  else {
    return obj.constructor === type;
  }
}

function check() {
  if ((arguments.length % 2) !== 0) {
    throw new Error("Too few arguments specified; checks must be passed in pairs.");
  }
  var l = arguments.length / 2, i = 1, o = 0;
  do {
    if (!typeCheck(arguments[o], arguments[o + 1])) {
      throw new Error("Type does no match, noooooo!");
    }
    o += 2;
    i += 1;
  } while (i <= l);
}

type-check.js

function getType(obj) {
  if (obj === undefined) {
    return undefined;
  }
  else if (obj === null) {
    return null;
  }
  else {
    switch(Object.prototype.toString.apply(obj)) {
      case '[object Array]':    return Array;     break;
      case '[object Date]':     return Date;      break;
      case '[object RegExp]':   return RegExp;    break;
      case '[object Function]': return Function;  break;
      case '[object Number]':   return Number;    break;
      case '[object String]':   return String;    break;
      default: return Object; break;
    }
  }
}

function typeCheck(obj, type) {
  if (type === undefined || type === null) {
    return obj === type;
  }
  else {
    return obj.constructor === type;
  }
}

function quickTest() {
  console.log("Function: ", typeCheck(function(){}, Function));
  console.log("String: ", typeCheck('blah', String));
  console.log("Number: ", typeCheck(1, Number));
  console.log("Object: ", typeCheck({}, Object));
  console.log("Array: ", typeCheck([], Array));
  console.log("Date: ", typeCheck(new Date(), Date));
  console.log("RegExp: ", typeCheck(/\w+/, RegExp));
  console.log("undefined: ", typeCheck(undefined, undefined));
  console.log("null: ", typeCheck(null, null));

  // Check a custom type
  var Person = function() {};
  var p = new Person()
  console.log("Person: ", typeCheck(p, Person));
}