Array as protected Object property in JavaScript

Question

I'm trying to build a JavaScript constructor which has an array as a property with read-only access:

var Word = function() {
  var _occurrences = [];
  Object.defineProperties(this, {
    "occurrences": {
      get: function() {
        return _occurrences;
      }
    },
    "addOccurence": {
      value: function(occ) {
        _occurrences.push(occ);
      }
    }
  });
};

The array itself is a private variable with a get-er pointing to it.

var myWord = new Word();
myWord.addOccurrence(123);
var occ = myWord.occurrences;

All works fine.

myWord.occurrences = [];

Is blocked, as it should be. But surprisingly, this works:

myWord.occurrences.push(321);

Protecting a property keeps it from new assignments, but not from write access through Array methods - even though it is only accessed through a getter. That makes Object.defineProperty() rather pointless to me.

Object.freeze() / Object.seal() is not an option as I need write access for my addOccurrences() method.

Any ideas? Have I overlooked something?

Quentin · Accepted Answer · 2014-06-19 13:36:24Z

4

JavaScript only provides you with references to objects (including arrays). When you return _occurrences, you return a reference to the array, so you can manipulate it.

If you want to prevent that, return a copy of the array instead.

return _occurrences.concat();

answered Jun 19, 2014 at 13:36

Quentin

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5 Comments

wortwart Over a year ago

That was the one thing I didn't think about ... thanks a lot!

stefferd Over a year ago

But should it be _occurrences.copy()? contat is there to concatinate arrays

Quentin Over a year ago

@stefferd — Concatenating an array to nothing gives you a copy.

stefferd Over a year ago

@Quentin thanks, but why not just use the copy function

Quentin Over a year ago

Because [].copy(); TypeError: [].copy is not a function

stefferd · Accepted Answer · 2014-06-20 08:29:32Z

1

If you want the object private you should write it like this:

(function(window) {
  var _occurrences = [];

  function Word() { /*empty constructor*/ }

  Word.prototype.get = function() {
    return _occurrences;
  };
  Word.prototype.add = function(value) {
    _occurrences.push(value);
  };

  window.Word = Word
})(window);

This way the scope is only available inside the created instance of the object. The value of the _occurences in the instance can not be overriden.

A created an example of this solution: http://jsfiddle.net/fxxaB/2/

But if you want the array accessed outside of the array i would recommend the following solution add a function like the example below:

Word.prototype.instance = function() {
  return _occurences.copy();
}

This way the array is accesible but the values can still not be manipulated.

edited Jun 20, 2014 at 8:29

answered Jun 19, 2014 at 13:45

stefferd

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1 Comment

wortwart Over a year ago

Yes, channelling access to the array through a custom-built getter function indeed protects the array. I would use this only as a fallback solution though, as I prefer to read access the array from outside with obj.myArray or obj.myArray[index] instead of obj.myArray().

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