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Question

Is it possible to make a javascript object property read-only? I want to set a property that cannot be modified...

Answer 1

It's possible, but expensive. You can do it by having a truly private member variable and then providing an accessor function:

var NiftyThing = function() {
    var trulyPrivateVariable;

    trulyPrivateVariable = 5; // For instance
    this.accessorFunction = function() {
        return trulyPrivateVariable;
    }
};

That works because the accessor function is a closure over the var. The cost is that each instance has its own copy of the accessor function.

EDIT: Usage:

var n = new NiftyThing();
alert(n.trulyPrivateVariable);
// Alerts "undefined"
alert(n.accessorFunction());
// Alerts "5"

See Private Member Variables in JavaScript for more.

Answer 2

You can implement something like this, making use of Object.defineProperty() :

function blockProperties(object, properties) {
    "use strict";
    // If not properties passed, then use the already defined ones:
    if (typeof properties === "undefined") {
        properties = object;
    }
    // Loop trough the properties
    for (var property in properties) {
        if (properties.hasOwnProperty(property)) {
            // Make property read-only
            Object.defineProperty(object, property, {
                value: properties[property],
                writable: false,
                configurable: false,
                enumerable: false
            });
        }
    }
    return object;
}

var someObject = {};

blockProperties(someObject, {
    propertie1: "someValue",
    propertie2: "someOtherValue"
});

someObject.propertie1 = "this doesn't change anything";

console.log(someObject.propertie1); // Output: "someValue"

// Because "window" is also an object, you can set an only-read global var:
blockProperties(window, {
    onlyReadVariable: "onlyReadValue"
});

Answer 3

I agree with the answer, and would like to note that some JavaScript frameworks like bob.js support such built-in mechanisms:

var obj = { };
//declare read-only property.
bob.prop.namedProp(obj, 'name', 'Bob', true);
//declare read-write property.
bob.prop.namedProp(obj, 'age', 1);

//get values of properties.
console.log(bob.string.formatString('{0} is {1} years old.', obj.get_name(), obj.get_age()));
//set value of read-write property.
obj.set_age(2);
console.log(bob.string.formatString('Now {0} is {1} years old.', obj.get_name(), obj.get_age()));

//cannot set read-only property of obj. Next line would throw an error.
// obj.set_name('Rob');

//Output:
//========
// Bob is 1 years old.
// Now Bob is 2 years old.

However, if you have special needs regarding property, such as specific get accessor implementation needs, then better define a function which gets the value as you need it.

- Tengiz