test_hello.js

var assert = require('assert');
var fs = require('fs');
var util = require('util');
var vm = require('vm');
var sinon = require('sinon');

var source = fs.readFileSync('./to_call.js').toString();

var script = new vm.Script(source);

var sandbox = {};
var context = new vm.createContext(sandbox);
script.runInContext(context);

var stubfakeFunction = sinon.stub(context, 'fakeFunction');

//console.log(util.inspect(sandbox, false, null));

it('hello from vm', function() {
  assert.equal(sandbox.hello() , 'hello world');
});

to_call.js

var fakeFunction = function () {
  throw 'fakeFunction was called';
};

var hello = function() {
  fakeFunction();
  return 'hello world';
};