Question

Javascript Code

var a = {};
a.test += 1; //NaN
++a.test; //NaN

Instead

var a = {};
a.test = 0;
++a.test; //1
a.test += 1; //2

I wonder if there could be anyway that can make first code sample work the same as second, i.e without an explicit assignment to 0. As in assigning default value for any property of an object to 0 instead undefined. I’m trying to do this in node.js. So, no problem of cross browser things and old ECMA Specs.

var i;
for(i = 0; i<10; i++) {
   if(a.test) {
     ++a.test;
   } else {
     a.test = 0;
     ++a.test;
   }
   //a.test = a.test || 0; (1)
   //++a.test;
}

If it is possible then the inner if/else or the assignment statement(1) in the above code can be eliminated.

Answer 1

There’s no way to do this for any arbitrary undefined property.

For a known property name, there is a way, but DO NOT USE IT !! ¹

Object.prototype.test = 0;

This will give every object an implicit .test property with that value in it, and the first time you attempt to modify it the result will be stored in your own object:

> Object.prototype.test = 0
> a = {}
> a.test++
> a.test
1

---

¹ Adding stuff to Object.prototype will break stuff, including for (key in obj)