Dynamic Getters and Setters - Why doesn't this work?

Question

I'm trying to create an object that builds its own getters and setters dynamically:

function Person( properties ) { // 'properties' is an object literal

    this._private = properties; // private by convention

    for ( key in this._private ) {

        this[key] = function() {

            return this._private[key];
        }
    }
}

I hoped, this would create something like this:

var jack = new Person({

    working:true,
    age:33,
    gender:'male'
});

jack.working() --> true
jack.age() --> 33
jack.gender() --> 'male'

The problem is, it always returns 'male', like so:

jack.working() --> 'male'
jack.age() --> 'male'
jack.gender() --> 'male'

What am I missing? Oh, and this is just a proof of concept. I know this is not a perfect solution for creating getters and setters in JavaScript.

I know this is not your question, and you might already be aware, but it would improve the code if you put a 'var' in the for loop, i.e.: for ( var key in this._private ) { ... }. Otherwise the 'key' variable will be created in your global scope (or even worse: any existing variable will be overwritten). — Maate, Nov 27 '12 at 20:54

Andrew Whitaker · Accepted Answer · 2012-11-27T21:04:12.693

7

You've got a classic scoping issue. Create a new function to create scope around key:

function Person(properties) { // 'properties' is an object literal
    var key;

    this._private = properties; // private by convention
    this.buildGetter = function (key) {
        this[key] = function () {
            return this._private[key];
        }
    };
    for (key in this._private) {
        this.buildGetter(key);
    }
}

Example: http://jsfiddle.net/SEujb/

edited Nov 27 '12 at 21:04

answered Nov 27 '12 at 20:36

Andrew Whitaker

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Explanation http://stackoverflow.com/questions/111102/how-do-javascript-closures-work – maxwell Nov 27 '12 at 20:43
Dang, I was completely missing the scoping pitfall. – Kriem Nov 27 '12 at 20:44
So he does not need to write `return this`? :O – Barney Szabolcs Nov 27 '12 at 22:17
1

@BarnabasSzabolcs Not sure what you mean? – Kriem Nov 28 '12 at 09:28
I meant at the end of function Person. – Barney Szabolcs Nov 28 '12 at 14:28

score 3 · Answer 2 · answered Nov 27 '12 at 20:51

3

Everybody says eval is evil, but sometimes it can't be avoided:

function Person( properties ) { // 'properties' is an object literal

    this._private = properties; // private by convention

    for ( key in this._private ) {

        eval("this[key] = function() { return this._private['"+key.toString()+"'];}");
    }
}

jsFiddle: http://jsfiddle.net/pitaj/9sWbe/1/

answered Nov 27 '12 at 20:51

PitaJ

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Although the answer by AndrewWhitaker is probably better for this instance. – PitaJ Nov 27 '12 at 20:54
2

There's absolutely no need to use `eval` here. – Andrew Whitaker Nov 27 '12 at 20:56
@AndrewWhitaker I never said there was. I said there may be some instances where there is no other way. – PitaJ Nov 27 '12 at 20:58

Dynamic Getters and Setters - Why doesn't this work?

2 Answers2