p4-require

require function for browser

Just include it on your page:

<script src="https://unpkg.com/p4-require@latest></script>

And you get a require function which you can use like you would in Node.js, you just need to specify the exact versions of the packeges you want to use inline using @x.x.x syntax:

var Backbone = require('backbone@1.3.3');

var User = Backbone.Model.extend({});
var user = new User({name: 'Ivan'});

console.log('User name is ' + user.get('name'));

See JsFiddel example.