Blockchain (I think?? maybe there needs to be more to be real but it's something)

chain.js

const hashMaker = require("hash.js");

const hashString = string =>
  hashMaker
    .sha256()
    .update(string)
    .digest("hex");

const findBlockWithHashPrefix = (signedGenerator, prefix) => {
  let nonce = 0;
  while (true) {
    nonce++;
    const signed = signedGenerator(nonce);
    const hash = hashString(signed);
    if (hash.slice(0, prefix.length) == prefix) {
      return { signed, hash };
    }
  }
};

const makeSigned = (chain, payload) => nonce => {
  return JSON.stringify({
    lastBlockHash: chain[chain.length - 1].hash,
    payload,
    nonce
  });
};

const addBlock = (chain, payload) => {
  const signedGenerator = makeSigned(chain, payload);
  return [...chain, findBlockWithHashPrefix(signedGenerator, "0")];
};

const verifyChain = chain => {
  return chain.every((block, idx) => {
    return (
      verifyBlockHashMatchesContent(block) &&
      verifyBlockContainsPreviousHash(block, chain[idx - 1])
    );
  });
};

const verifyBlockHashMatchesContent = block => {
  return hashString(block.signed) === block.hash;
};

const verifyBlockContainsPreviousHash = (block, prevBlock) => {
  // If prevBlock is undefined, we're the genesis block and we can do what we like
  if (prevBlock === undefined) {
    return true;
  }
  return JSON.parse(block.signed).lastBlockHash === prevBlock.hash;
};

module.exports = {
  addBlock,
  verifyChain
};

chain.test.js

const hashMaker = require("hash.js");
const Chain = require("./chain");

const GENESIS_BLOCK = {
  hash: "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
  signed: ""
};

describe("addBlock", () => {
  test("adds a block", () => {
    const chain = Chain.addBlock([GENESIS_BLOCK], "hi!");
    expect(chain).toEqual([
      GENESIS_BLOCK,
      {
        hash:
          "0574f789c3b7f5cbd10470f2fdec7d9c590d51e8663d69f96dcf0a775b263449",
        signed:
          '{"lastBlockHash":"e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855","payload":"hi!","nonce":2}'
      }
    ]);
  });
});

describe("verifyChain", () => {
  test("verifies a correct chain", () => {
    const correctChain = Chain.addBlock([GENESIS_BLOCK], "hi!");
    expect(Chain.verifyChain(correctChain)).toBe(true);
  });

  test("invalidates an incorrectly hashed chain", () => {
    const correctChain = [
      GENESIS_BLOCK,
      {
        hash: "badhash",
        signed:
          '{"lastBlockHash":"e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855","payload":"hi!","nonce":2}'
      }
    ];
    expect(Chain.verifyChain(correctChain)).toBe(false);
  });

  test("invalidates an incorrectly chained chain", () => {
    const correctChain = [
      GENESIS_BLOCK,
      {
        hash:
          "0574f789c3b7f5cbd10470f2fdec7d9c590d51e8663d69f96dcf0a775b263449",
        signed: '{"lastBlockHash":"badhash","payload":"hi!","nonce":2}'
      }
    ];
    expect(Chain.verifyChain(correctChain)).toBe(false);
  });
});

package.json

{
  "name": "bchain",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "hash.js": "^1.1.3",
    "jest": "^22.0.3"
  },
  "scripts": {
    "test": "jest"
  }
}