newを使わない場合はこう。

var x = {};
(function(obj) {
  var maxLength;
  function getMaxLength() { return maxLength; }
  function setMaxLength(val) { maxLength = val; }
  obj.getMaxLength = getMaxLength;
  obj.setMaxLength = setMaxLength;
})(x);

console.log(x.maxLength);    // => undefined
x.setMaxLength(100);
console.log(x.getMaxLength()); // => 100
console.log(x.maxLength);    // => undefined
x.maxLength = 200;        // => can't change Foo's property
console.log(x.getMaxLength()); // => 100
try {
  console.log(x.getMaxlength());
} catch (e) {
  console.log(e.message);
}