git.js

// https://kushagra.dev/blog/build-git-learn-git/

function Git(name) {
  this.name = name;
  this.lastCommitId = -1;
  this.branches = [];
  const master = new Branch("master", null);
  this.branches.push(master);
  this.HEAD = master;
}

Git.prototype.commit = function (message) {
  const commit = new Commit(++this.lastCommitId, this.HEAD.commit, message);
  this.HEAD.commit = commit;
  return commit;
};

Git.prototype.log = function () {
  const history = [];
  let { commit } = this.HEAD;
  while (commit) {
    history.push(commit);
    commit = commit.parent;
  }
  return history;
}

Git.prototype.checkout = function (branchName) {
  // Loop through all branches and see if we have a branch
  // called `branchName`.
  for (const branch of this.branches) {
    if (branch.name === branchName) {
      console.log("Switched to existing branch: " + branchName);
      this.HEAD = branch
      return branch;
    }
  }
  // If branch was not found, create a new one.
  const newBranch = new Branch(branchName, this.HEAD.commit);
  this.branches.push(newBranch);
  this.HEAD = newBranch;
  console.log("Switched to new branch: " + branchName);
  return newBranch;
};

function Commit(id, parent, message) {
  this.id = id;
  this.parent = parent;
  this.message = message;
}

function Branch(name, commit) {
  this.name = name;
  this.commit = commit;
}

// Maps the array of commits into a string of commit ids.
// For [C2, C1,C3], it returns "2-1-0"
function historyToIdMapper(history) {
  var ids = history.map(function(commit) {
    return commit.id;
  });
  return ids.join("-");
}

var repo = new Git("test");
repo.commit("Initial commit");
repo.commit("Change 1");


console.assert(historyToIdMapper(repo.log()) === "1-0"); // Should show 2 commits.

repo.checkout("testing");
repo.commit("Change 3");

console.assert(historyToIdMapper(repo.log()) === "2-1-0"); // Should show 3 commits.

repo.checkout("master");
console.assert(historyToIdMapper(repo.log()) === "1-0"); // Should show 2 commits. Master unpolluted.

repo.commit("Change 3");
console.assert(historyToIdMapper(repo.log()) === "3-1-0"); // Continue on master with 4th commit.