gistfile1.js

var mock = require('my-awesome-mock-module')(beforeEach, afterEach);

describe('foo', function() {
  var fn = mock(foo, 'bar', [] /* default return value */, true /* async */);
  // foo.bar = function(a, b, c) { c(null, []); }
  // override in beforeEach hook, restore original in afterEach
  it('should qux', function() {
    fn([1]); // foo.bar now returns [1] (node async style)
    fn(new Error()); // foo.bar now fails
  });
  
  // OR
  
  var fn = mock(foo, 'bar', function(a, b, c) { a.should.be.true; c(); });
  var fn = mock(foo, 'bar', function(a, b) {}, true); // assert function signature to match with the original function (fail on API changes)
});