Mockery useCleanCache gotcha

bar.js

module.exports = function hello() {
  console.log("Byebye");
}

foo.js

module.exports = function hello() {
  console.log("Hello");
}

mockery_gotcha.js

var mockery = require('mockery-next');
var foo = require('./foo');

foo();

mockery.registerSubstitute('./foo', "./bar");
mockery.enable({
  useCleanCache: true,
  warnOnUnregistered: false
});

// foo = require('./foo');
foo();

mockery.disable();

foo();

output.md

With line 12 commented

Hello
Hello
Hello

With line 12 uncommented

Hello
Byebye
Byebye

TL;DR

useCleanCache is no magic option. Unless a specific call to require is made again, the module will still point to its former version rather than throwing an error.