From the node.js documentation:

Modules are cached after the first time they are loaded. This means (among other things) that every call to require('foo') will get exactly the same Object returned, if it would resolve to the same file.

Is there a way to invalidate this cache? i.e. for unit testing, I'd like each test to be working on a fresh object.

You can always safely delete an entry in require.cache without a problem, even when there are circular dependencies. Because when you delete, you just delete a reference to the cached Module Object, not the module object itself, the module object will not be GCed because in case of circular dependencies, there is still a object referencing this module object. suppose you have a script a.js:

var b=require('./b.js').b;
exports.a='a from a.js';
exports.b=b;

and a script b.js:

var a=require('./a.js').a;
exports.b='b from b.js';
exports.a=a;

when you do:

var a=require('./a.js')
var b=require('./b.js')

you will get:

> a
{ a: 'a from a.js', b: 'b from b.js' }
> b
{ b: 'b from b.js', a: undefined }

now if you edit your b.js:

var a=require('./a.js').a;
exports.b='b from b.js. changed value';
exports.a=a;

and do:

delete require.cache[require.resolve('./b.js')]
b=require('./b.js')

you will get:

> a
{ a: 'a from a.js', b: 'b from b.js' }
> b
{ b: 'b from b.js. changed value',
  a: 'a from a.js' }