Create a page that allows the following logic: Step 1. Define a field set for an arbitrary table. This field set has the mandatory field id and as many additional fields as the user desires. Step 2. Add records. The user should be presented with a table of some kind. The user should be able to add records to the table and to edit any record existent in the table. The user should not be allowed to commit a record if the id is non unique amongst the other records in the table. Step 3. Export the records. The user should be able to export the records in json format. This page should be runnable by a browser without access to the internet, so please keep all dependencies with the page to minimize any dependency loading difficulties. Feel free to use any library you wish as long as it does not solve the entirety of the problem. For example, jQuery, angular, Backbone, underscore, etc. are all fine but a table editing jQuery plugin would not be fine.
