Question

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 good! Where'd ninja.yell go?" );
}

Answer 1

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.