Is there any way to prevent replacement of JavaScript object properties?

Question

I would like to make an object's structure immutable, preventing its properties from being subsequently replaced. The properties need to be readable, however. Is this possible?

I'm sure there are no language features (along the lines of final in Java and sealed in C#) to support this but wondered whether there might be another mechanism for achieving the same result?

I'm looking for something along these lines:

var o = {
    a: "a",
    f: function () {
        return "b";
    }
};

var p = o.a;        // OK
o.a = "b";          // Error
var q = o.f();      // OK
o.f = function () { // Error
    return "c"; 
};

Answer 1

the best thing you can do is hide your properties inside of a closure.

var getMap = function(){
  var hidden = "1";
  return {
    getHidden : function() { return hidden; }
  }
}

var f = getMap ();

alert(f.getHidden());

I took a stab at it. In the above code you will need to not just return hidden but copy it into a new object perhaps. maybe you can use jquery's extend to do this for you, so you will be returning a new object, not the reference. This may be completely wrong though =)

Answer 2

ECMAScript 5 will have seal() and freeze(), but there's no good way to do this with current JavaScript implementations.

Source.

Answer 3

As mkoryak said, you can create a closure to hide properties

function Car(make, model, color) {
    var _make = make, _model = model, _color = color; 

    this.getMake = function() {
        return _make;
    }

}

var mycar = new Car("ford", "mustang", "black");

mycar.getMake(); //returns "ford"
mycar._make; //error

Answer 4

Using var in an object constructor will create a private variable. This is essentially a closure. Then you can create a public function to access/modify it. More information and examples available on Private Members in Javascript by Douglas Crockford.