Question

I examined a javascript exception in Google Chrome.

And I noticed the functions get message, get stack, set message, and set stack. I tried catching this exception and running alert(e.get_message()); only to get an error. I also tried to run alert(e.get message());, which obviously returned another error due to the space.

What are these mysterious methods, and how does a developer call them?

Answer 1

They’re property accessors. They’re effectively functions that run when you get or set the property.

e.message; // getter

e.message = "foobar"; // setter

Using property accessors, these do more than just a simple get and set of the property value. They can run code that was established in the object’s property descriptors, so that the property access can have side-effects.

Example:

var o = Object.create(Object.prototype, {
    foobar: {
        get: function() { return "getter"; },
        set: function(val) { alert("setter " + val); }
    }
});

o.foobar;  // "getter"
o.foobar = "raboof";  // alerts "setter raboof"

---

To see the property descriptors set for a given property, use Object.getOwnPropertyDescriptor…

console.dir(Object.getOwnPropertyDescriptor(e, "message"));

Object
    configurable: true
    enumerable: false
    get: function getter() { [native code] }
    set: function setter() { [native code] }

Note that these techniques require an ECMAScript 5 supported implementation.