bail_1.js

var foo = {
  hi: 'asdf'  
};

function bro(key) {
  return foo[key];
}

bro('hi');
%optimizeFunctionOnNextCall(bro);
bro('hi');

bail_2.js

var foo = {
  hi: 'asdf'  
};

function bro(key) {
  return foo[key];
}

bro('hi');
bro(1);
%optimizeFunctionOnNextCall(bro);
bro('hi');

no_bail.js

var foo = {
  hi: 'asdf'  
};

function bro(key) {
  return foo[key];
}

bro('hi');
bro(true); // apparently this provides sufficient type information for opt, but nothing else?
%optimizeFunctionOnNextCall(bro);
bro('hi');

node stable doesn't bail, node master (latest) and v8 master bail.

the bailout:

  ;;; deoptimize: Insufficient type feedback for keyed load

https://code.google.com/p/v8/source/browse/trunk/src/hydrogen.cc#5764

also related: https://github.com/v8/v8/blob/master/src/hydrogen.cc#L6505-L6508