Can Read-Only Properties be Implemented in Pure JavaScript?

Question

Looking at the mozilla documentation, looking at the regular expression example (headed "Creating an array using the result of a match"), we have statements like:

input: A read-only property that reflects the original string against which the regular expression was matched.

index: A read-only property that is the zero-based index of the match in the string.

etc... is it possible to create your own object in JavaScript which will have read-only properties, or is this a privilege reserved to built-in types implemented by particular browsers?

Answer 1

I'm not sure what they mean by read-only; I'm able to do this (at least in Safari):

var myRe = /d(b+)(d)/i;
var myArray = myRe.exec("cdbBdbsbz");
myArray.input = 'something else';
alert(myArray.input); // displays 'something else'

Answer 2

It is possible to have read-only properties in JavaScript which are available via getter methods. This is usually called the 'Module' pattern.

The YUI blog has a good writeup of it: http://yuiblog.com/blog/2007/06/12/module-pattern/

Snippet from the post:

YAHOO.myProject.myModule = function () {

//"private" variables:
var myPrivateVar = "I can be accessed only from within YAHOO.myProject.myModule.";

//"private" method:
var myPrivateMethod = function () {
 YAHOO.log("I can be accessed only from within YAHOO.myProject.myModule");
}

return  {
 myPublicProperty: "I'm accessible as YAHOO.myProject.myModule.myPublicProperty."
 myPublicMethod: function () {
  YAHOO.log("I'm accessible as YAHOO.myProject.myModule.myPublicMethod.");

  //Within myProject, I can access "private" vars and methods:
  YAHOO.log(myPrivateVar);
  YAHOO.log(myPrivateMethod());

  //The native scope of myPublicMethod is myProject; we can
  //access public members using "this":
  YAHOO.log(this.myPublicProperty);
 }
};

}(); // the parens here cause the anonymous function to execute and return

Answer 3

In Firefox, Opera 9.5+, and Safari 3+, and Chrome (but not Internet Explorer) you can define getter and setter properties. If you only define a getter, it effectively creates a read-only property. You can define them in an object literal or by calling a method on an object.

var myObject = {
    get readOnlyProperty() { return 42; }
};

alert(myObject.readOnlyProperty); // 42
myObject.readOnlyProperty = 5;    // Assignment is allowed, but doesn't do anything
alert(myObject.readOnlyProperty); // 42

If you already have an object, you can call __defineGetter__ and __defineSetter__:

var myObject = {};
myObject.__defineGetter__("readOnlyProperty", function() { return 42; });

Of course, this isn't really useful on the web because it doesn't work in Internet Explorer.

You can read more about it from John Resig's blog or the Mozilla Developer Center.

Answer 4

Here's link to Douglas Crockford's page on "Private Members in Javascript"....it would seem to me these would be read only if only getter methods are supplied, and no setters:

http://javascript.crockford.com/private.html

Answer 5

With any javascript interpreter that implements ECMAScript 5 you can use Object.defineProperty to define readonly properties. In loose mode the interpreter will ignore a write on the property, in strict mode it will throw an exception.

Example from ejohn.org:

var obj = {};
Object.defineProperty( obj, "value", {
  value: true,
  writable: false,
  enumerable: true,
  configurable: true
});