Eli Grey

ECMAScript 5 accessors

A while ago, I created a JavaScript library named Xccessors, which implemented the legacy non-standard accessor (getter and setter) methods in IE8. I initially created two different libraries and decided that the library that implemented the legacy methods would be more useful at the time due to no changes needed in a JavaScript program’s code to add accessor support for IE. I never released the second one that implements the ECMAScript 5 (formerly 3.1) standard accessor methods so I’m releasing it now. There can’t be two different libraries with the same “Xccessors” name, so I am also renaming the libraries accordingly to what they implement. The new names are Xccessors Legacy and Xccessors Standard.

Here are two examples of using Object.defineProperty and Object.getOwnPropertyDescriptor:

Using accessors

(function() {
// this creates a document.foo accessor
    var foo = 0;
    Object.defineProperty(document, "foo", {
        get: function() { return foo },
        set: function(x) { return foo += x }
document.foo = 5;
(document.foo = 4) === 9;
//Object.getOwnPropertyDescriptor(document, "foo") is {set:[...], get:[...]}

Setting a property

// this is the equivalent of window.foo = "bar";
Object.defineProperty(window, "foo", {value: "bar"});
window.foo === "bar";
Object.getOwnPropertyDescriptor(window, "foo").value === "bar";
window.x = 0;
Object.getOwnPropertyDescriptor(window, "x").value === 0;


The addition of Object.getPrototypeOf in JavaScript 1.8.1 reminded me of John Resig’s Object.getPrototypeOf implementation that uses constructor property even if it has been modified. If the constructor property has been modified, it can be reset by deleting the property. An easy way to detect if a property like constructor has been modified is check if the object’s hasOwnProperty method returns true when passed "constructor". This could have been modified too, so the safest bet it to call Object.prototype.hasOwnProperty. Though the thing is, you can’t even trust that, so you just have to assume that it isn’t function hasOwnProperty() false;.
Now I bet you’re wondering, why I should go through all this trouble to check if the constructor property is modified. This is because just storing the constructor, then deleting, checking, and restoring it will set the constructor property if it wasn’t already set, which can mess with iterators. Of course you could just delete it and hope that if it was set, it wasn’t important, but that won’t always end up well.

You can download my implementation at gist.github.com/154398.