Function call fails when object set to null

Can anyone explain why the Javascript from these slides fail?

var ninja = {
  yell: function(n){
    return n > 0 ? ninja.yell(n-1) + "a" : "hiy";
  }
};
assert( ninja.yell(4) == "hiyaaaa", "A single object isn't too bad, either." );

var samurai = { yell: ninja.yell };
var ninja = null;

try {
  samurai.yell(4);
} catch(e){
  assert( false, "Uh, this isn't goo开发者_开发百科d! Where'd ninja.yell go?" );
}

The recursive function call uses the ninja reference inside yell(), and when you set that to null it can't call ninja.yell anymore because ninja is null.

Change:

return n > 0 ? ninja.yell(n-1) + "a" : "hiy";

To:

return n > 0 ? this.yell(n-1) + "a" : "hiy";

And it works.

This reference is still maintained:

var samurai = { yell: ninja.yell };

So that call to .yell() is fine, however this one isn't:

return n > 0 ? ninja.yell(n-1) + "a" : "hiy";

That call's actually referencing ninja which is now null, and null.yell() isn't a function :)

Here's a modified version that shows the problem.

You wrote ninja = null before calling the function.
Javascript functions capture outer variables by reference, so the ninja identifier inside the function refers to the current value of the variable.

Samurai.yell is a "copy" of the function from ninja. In that function, it referenced ninja, which no longer existed when Samurai.yell was executed.