To fix your issue, i have made few changes in bla.js and it is working,
var foo= function (req, res, next) {
console.log('inside foo');
return ("foo");
}
var bar= function(req, res, next) {
this.foo();
}
module.exports = {bar,foo};
and no modification in app.js
var bla = require('./bla.js');
console.log(bla.bar());