Configuring TeX macros in default.js vs local.js

Hi,

Can someone please explain why the syntax for defining macros is different
in different files? In default.js we have:

Macros: {
foo: 'bar',

But in local.js we have:

TEX.Macro('foo', 'bar');

I'm guessing this has something to do with WHEN that piece of code is run,
but I haven't quite got it. I'm working with the mathjax-node code, and I'm
finding the second form works, but not the first.

Thanks,
Fred