How to make dynamic getter with defineProperty in JavaScript?

Question

I want to define dynamic getter functions with defineProperty in JavaScript like below. In order to make a readonly function, I want to use defineProperty. This enables me to throw an Exception in the setter function.

However the getter function won't be functional. I thought that this getter returns the any properties of the obj dynamically. But it wasn't. It always returns obj["three"], last property. Is there any methods to make dynamic getter which returns appropriate property in JavaScript?

var obj = {"one":1, "two":2, "three":3};
var cloned = {};

for (var prop in obj) 
{
    var getter = makeGetter(prop);
    Object.defineProperty(cloned, prop, 
    {
        set: function() 
        {
            throw new UnableRewriteException('original cannot be rewrite');
        },
        get: function() 
        {
            return obj[prop]
        },
        enumerable: true
    });
}

This again looks like the classic loop-closure problem, remember that `var` will be function level not block level. — Paul S., Dec 22 '13 at 14:22
possible duplicate of [Javascript closure inside loops - simple practical example](http://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example) — Paul S., Dec 22 '13 at 14:23

score 2 · Accepted Answer · answered Dec 22 '13 at 14:41

As @paul-s mentioned you have a problem with a closure inside your loop. A simple fix:

var obj = {"one":1, "two":2, "three":3};
var cloned = {};

function makeReadOnlyProperty(cloned, obj, prop) {
    Object.defineProperty(cloned, prop, 
    {
        set: function() 
        {
            throw new UnableRewriteException('original cannot be rewrite');
        },
        get: function() 
        {
            return obj[prop]
        },
        enumerable: true
    });
}

for (var prop in obj) 
{
    makeReadOnlyProperty(cloned, obj, prop);
}

Thank you so much. My problem was solved and I can obtain deep understanding of JavaScript closure. :-) — Kai Sasaki, Dec 30 '13 at 03:24

How to make dynamic getter with defineProperty in JavaScript?

1 Answers1