Comments on: I Object

By: Neil

Neil — Thu, 31 May 2007 13:23:57 +0000

hasOwnProperty can be avoided by using the following:

if(myList[word] && (myList[word] !== {}[word]){
…
}

This would also allow for the myList object to inherit non-owned properties from the prototype chain.

By: Julien Royer

Julien Royer — Sun, 13 May 2007 23:28:11 +0000

If you use a JavaScript object as a general hash map (i.e. with undetermined keys), it is even easier to make your program fail:

var obj = {};

obj["hasOwnProperty"] = “coconut”;

alert(obj.hasOwnProperty(“a”));

By: Douglas Crockford

Douglas Crockford — Sun, 13 May 2007 22:34:24 +0000

Daniel brings up an interesting case. __proto__ is a non-standard deviation from the ECMAScript standard that can be found in the Mozilla browsers. It can cause good programs to fail.

By: Julien Royer

Julien Royer — Sun, 13 May 2007 22:34:19 +0000

I agree with Daniel. The use of a raw abject as a general hash map in JavaScript can not be totally bulletproof.

By: Daniel

Daniel — Sun, 13 May 2007 11:45:15 +0000

hasOwnProperty isn’t quite bulletproof, because on some browsers myList.hasOwnProperty(“__proto__”) will return true – since all objects have a __proto__ member pointing to their prototype. And then, if you then make any changes to myList["__proto__"], you’ll be messing with the object prototype. Nice.

Of course, “__proto__” doesn’t turn up that often. But this can lead to odd and subtle bugs.

By: Tino Zijdel

Tino Zijdel — Fri, 11 May 2007 20:26:13 +0000

I wasn't really surprised when you started using hasOwnProperty() in json.js ;) Although some people still consider prototyping Object 'verboten' I do think there are some valid reasons to do so, like when you want to backport some general object methods to older browsers. toJSONString() is a good example because it will probably become a standard method in future javascript, so the only way to have the same support in older (now current) browsers will be to prototype this method on Object. I once wrote a blogpost about that which also contains a nice snippet for browsers that do not natively support hasOwnProperty() and which ironically and necessarily uses Object.prototype: .toJSONString() and Object.prototype

By: Geoff Moller

Geoff Moller — Fri, 11 May 2007 18:16:59 +0000

It's always refreshing to work with a language that constantly forces you to examine it's warts in order for you work with it correctly. :) It should be noted that Prototype's Ruby-inspired collection facilities (Enumerable, Array, and Hash) allow you to abstract this fact-checking to a single place. Of course, we will always have to pick our flavor of pain: the "break" functionality in Prototype is accomplished with a global hack. Additionally, the current uncompressed version of Prototype is now weighing in at ~ 95kb. Ahh, sweet sweet Javascript - you are a harsh mistress.


/* NOTE - console.log requires Firebug: http://www.getfirebug.com/ */

/* spits out all links' hrefs and all the wonderful native goodness */
console.log("Native");
var links = document.getElementsByTagName('a');
for(var link in links){
	console.log(link + ":" + links[link]);
}

console.log(" ");

/* spits out all links' hrefs */
/* REQUIRES PROTOTYPE */
console.log("Massaged");
links = $A(document.getElementsByTagName('a'));
links.each(function(link){
	console.log(link.href); 
});

By: Julien Royer

Julien Royer — Fri, 11 May 2007 12:13:06 +0000

@Matt : it doesn’t work. For example, if keys are added to the Objet constructor after the computation of “keysToIgnore”.

“hasOwnProperty” is not supported on IE 5.

By: Donzo

Donzo — Fri, 11 May 2007 00:28:35 +0000

So, seems that using members for a purpose never intended (and arguably in a way many developers would initially find confusing) gets you into trouble. No surprise there. Is it the language’s fault … or the coders?

By: Matt

Matt — Thu, 10 May 2007 22:26:34 +0000

A trick I use to detect what methods have been added to Array and Object, is to include this script right after my libraries. Then any time I use the for/in I ignore these keywords.

var keysToIgnore = function() {
var keys = [];
var arr = [], obj = {};
for (var k in arr) {keys.push(k);}
for (var k in obj) {keys.push(k);}
return keys;
}();