Defining read-only properties in JavaScript

Question

Given an object obj, I would like to define a read-only property 'prop' and set its value to val. Is this the proper way to do that?

Object.defineProperty( obj, 'prop', {
    get: function () {
        return val;
    }
});

The result should be (for val = 'test'):

obj.prop; // 'test'
obj.prop = 'changed';
obj.prop; // still 'test' since it's read-only

This method works btw: http://jsfiddle.net/GHMjN/
I'm just unsure if this is the easiest / smoothest / most proper way to do it...

Answer 1

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.

Answer 2

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'

Answer 3

In my case I needed an object where we can set its properties only once.
So I made it throw an error when somebody tries to change already set value.

class SetOnlyOnce {
    #innerObj = {}; // private field, not accessible from outside

    getCurrentPropertyName(){
        const stack = new Error().stack; // probably not really performant method
        const name = stack.match(/\[as (\w+)\]/)[1];
        return name;
    }

    getValue(){
        const key = this.getCurrentPropertyName();

        if(this.#innerObj[key] === undefined){
            throw new Error('No global param value set for property: ' + key);
        }

        return this.#innerObj[key];
    }

    setValue(value){
        const key = this.getCurrentPropertyName();

        if(this.#innerObj[key] !== undefined){
            throw new Error('Changing global parameters is prohibited, as it easily leads to errors: ' + key)
        }

        this.#innerObj[key] = value;
    }
}


class GlobalParams extends SetOnlyOnce {
    get couchbaseBucket() { return this.getValue()}
    set couchbaseBucket(value){ this.setValue(value)}

    get elasticIndex() { return this.getValue()}
    set elasticIndex(value){ this.setValue(value)}   
}

const _globalParams = new GlobalParams();

_globalParams.couchbaseBucket = 'some-bucket';
_globalParams.elasticIndex = 'some-index';

console.log(_globalParams.couchbaseBucket)
console.log(_globalParams.elasticIndex)

_globalParams.elasticIndex = 'another-index'; // ERROR is thrown here
console.log(_globalParams.elasticIndex)

Answer 4

Because of the old browsers (backwards compatibility) I had to come up with accessor functions for properties. I made it part of bob.js:

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.

I hope it helps.

Answer 5

10+ years after this question, ES6 is not a concern even in old browsers. If the object functionality can be satisfied by class construct with static members, then it can be implemented as

const obj = class {
  static #prop = "test";
  static get prop() { return obj.#prop }
};

console.log(obj.prop); // test
obj.prop = 'changed';
console.log(obj.prop); // test

Note that class constructs are special functions, not objects:

typeof obj; // function

Answer 6

I tried and it Works ...

element.readOnly = "readOnly"  (then .readonly-> true)
element.readOnly = ""  (then .readonly-> false)