#summary Simplify prototype chain setup.

= Introduction =

The GWT compiler generates standard Javascript-style class hierarchies with prototype chains used to represent parent-child relationships. This leads to a lot of generated code like this:

{{{
function Sub() {}
_ = Sub.prototype = new Super();
_.member1 = value1;
...
}}}

This could be made more terse with a structure like the following:

{{{
function $(sub, super) {
  return sub.prototype = new super();
}

function Sub() {}
_ = $(Sub, Super);
_.member1 = value1;
...
}}}

= Performance Impact =

Unfortunately, moving function invocations into the prototype chaining process is slower on every browser we've tested. The speed difference ranges from 4% on Chrome to 30% on Firefox 3. On the Android browser, it's slower by a whopping 45%. We need a more detailed investigation to determine how much of a practical difference it makes on real code, but because the speed difference affects application startup, we must tread carefully.

= Alternative Formulation =

An alternative formulation proposed recently would take the following form:

{{{
function $(superClass, members){
  var result = function() {};
  result.prototype = members;
  members.__proto__ = superClass.prototype;
  return result;
}

var myClass=$(superClass, {
  member1: value1,
  ...
});
}}}

This effectively avoids the `new` expression as well as the series of assignment statements used to initialize a class' members. It reportedly performs just as well as the original formulation. Unfortunately, the __proto__ field doesn't exist on Internet Explorer (at least not as of IE7), so in order to use it we would have to move knowledge of User-Agent into the compiler.