Question

Object.preventExtensions and Object.seal prevent unknown properties from being added to an object, but those attributions silently fail instead of throwing an error. Is there a way to force them to be errors?

var myObj = Object.seal({});
try{
    myObj.someProp = 17;
    console.log("I don't want to reach this message");
}catch(err){
    console.log("I want an error to occur instead.")
    console.log("Or at least get a warning somewhere.");
}

I tested this in Chrome 19 and Firefox 9. I wouldn’t mind browser-specific solutions either, since I would only need this during development.

Answer 1

If strict mode is an option, Object.seal seems to do the trick (at least in Firefox):

Attempting to delete or add properties to a sealed object, or to convert a data property to accessor or vice versa, will fail, either silently or by throwing a TypeError (most commonly, although not exclusively, when in strict mode code).

Works pretty good: http://jsfiddle.net/yVWr6/

(btw: works the same for preventExtensions())