How can I throw an error when accessing an object key that doesn't exist? [duplicate]

Question

This one's a long shot...

In Javascript, I was accessing an object attribute that I was certain existed, but I had a typo was in the name of the key, so was returning undefined and creating a bug.

How can I write code equivalent to the following, but that throws an error because the key does not exist?

var obj = {'myKey': 'myVal'},
    val = obj.myKye;

I'm trying to find a solution that doesn't require me writing a wrapper function that I use every time I want to access a member of an object. Is it possible? Is there another, 'stricter' technique in Javascript for accessing object attributes?

Answer 1

You can't...

If you want to be very careful hasOwnProperty will let you check if property is defined.

function GetSafe(obj, propertyName)
{
   if (obj.hasOwnProperty(propertyName)) return obj[propertyName];
   return "Unknown property:"+ propertyName; // throw or some other error reporting.
}
var obj = {'myKey': 'myVal'};
alert(GetSafe(obj, "myKey"));
alert(GetSafe(obj, "myKye"));

Answer 2

Try this:

function invalidKeyException(message) {
   this.message = message;
   this.name = "invalidKeyException";
}

var obj = {'myKey': 'myVal'},
val = obj.myKye;
if (val == undefined)
    throw new invalidKeyException("Key does not exist.");

JSFiddle: http://jsfiddle.net/r2MM4/

(Look at the console)