Defining read-only properties in JavaScript

You could instead use the writable property of the property descriptor, which prevents the need for a get accessor:

var obj = {};
Object.defineProperty(obj, "prop", {
    value: "test",
    writable: false
});

As mentioned in the comments, the writable option defaults to false so you can omit it in this case:

Object.defineProperty(obj, "prop", {
    value: "test"
});

This is ECMAScript 5 so won't work in older browsers.

In new browsers or node.js it is possible to use Proxy to create read-only object.

var obj = {
    prop: 'test'
}

obj = new Proxy(obj ,{
    setProperty: function(target, key, value){
        if(target.hasOwnProperty(key))
            return target[key];
        return target[key] = value;
    },
    get: function(target, key){
        return target[key];
    },
    set: function(target, key, value){
        return this.setProperty(target, key, value);
    },
    defineProperty: function (target, key, desc) {
        return this.setProperty(target, key, desc.value);
    },
    deleteProperty: function(target, key) {
        return false;
    }
});

You can still assign new properties to that object, and they would be read-only as well.

Example

obj.prop
// > 'test'

obj.prop = 'changed';
obj.prop
// > 'test'

// New value
obj.myValue = 'foo';
obj.myValue = 'bar';

obj.myValue
// > 'foo'