Bootstrap3 typehead ajax Dont know why it works

I am useing this plugin for an ajax auto complete feature https://github.com/bassjobsen/Bootstrap-3-Typeahead the bootstrap-3 type. The code below is working but I do not know why it works. Specifically how the the process and response parameter work. $(document).ready(function() { $('#typeahead-input').typeahead({ autoSelect: true, minLength: 1, delay: 400, source: function (query, process) { $.ajax({ url: '/api/location', data: {sstr: query}, dataType: 'json' }) .done(function(response) { // console.log(response) return process(response); }); } }); }); my json looks like this [ { "id": "123", "name": "Frederiksted", "state": "VI", "zip_code": "840" } ] What if i wanted to autocomplete on the zip_code field how would i do it? I have tried doing "response.zipcode" but it comes out as undefined