How to run test site

npm install
npm run start

Open link http://localhost:8080/ in internet browser.

Open debug console by F12.

See result log in main page and some debug info in console log.

How to rebuild callbackABI

Navigate to interfaces dir.

Then

tondev sol compile IFlexAPI.sol

It produces IFlexAPI.abi.json. Copy all from this file to callbackABI.js