Bracket notation with Object.hasOwnProperty?

Question

So I was reading through Javascript: The Definitive Guide and was reviewing this simple function:

function merge(o, p){
    for(prop in p){
        if(o.hasOwnProperty[prop]) continue;
        o[prop] = p[prop];
    }
    return o;
}

Seems simple enough, but when I run it it does not work unless I change o.hasOwnProperty[prop] to o.hasOwnProperty(prop).

This makes sense to me since hasOwnProperty is a method. Does this mean that this is just a mistake in the book? I'm reading the 6th edition, and this is printed on page 127 for those interested.

I suspect it is, but I just want to be sure there's not something strange about this function that I'm just not aware of that would make it work.

Answer 1

Does this mean that this is just a mistake in the book?

Yes. That's really surprising.

o.hasOwnProperty[prop] is valid code, just not what Flanagan meant to use there. (It looks for a property with the name from the prop variable on the hasOwnProperty function object.)

Please tell me he declared prop somewhere as well. It's not declared in your quoted code, thus making the code seem to fall prey to The Horror of Implicit Globals.