Blockchain Demo: Sean Han's toy chain lets you mine blocks and watch tampering break it

Sean Han's freeCodeCamp tutorial opens with Wikipedia's definition of a blockchain — 'a distributed database that is used to maintain a continuously growing list of records, called blocks' — and asks the obvious question: that sounds nice, but how does it work? His answer was not more diagrams but a working toy: an open-source command-line tool plus a browser app that lets readers run a real miniature blockchain themselves.

The mechanics are the lesson. Readers install Blockchain CLI and type a command like 'mine freeCodeCamp♥', then watch the app take the latest block's details and try nonce after nonce until a SHA-256 hash with the required leading zeros appears, linking the new block onto the chain. The article walks through each block's anatomy — index, timestamp, data, previous hash, nonce — then shows what happens if someone edits block A in a chain A→B→C: every later hash changes, every block becomes invalid, and only re-mining the whole chain could repair it.

The tutorial was published by freeCodeCamp in September 2017 and reached Hacker News, where it drew 156 points and 31 comments. The discussion shows the app working as intended: readers who had just been through the Bitcoin white paper asked for a part two on transactions, debated whether aliasing the Unix bc command was wise, and worked through orphaned blocks and 51% attacks — the toy had made the abstract concrete enough to argue about.

The demo did not disappear with the blog post. The browser version stayed online and kept evolving: in 2026 blockchaindemo.io is still live as an interactive visual demonstration of the blockchain data structure, and the tutorial's copy remains available through freeCodeCamp.