Usage: `addFileToGit(foo.txt, "some file contents")`. Uses the "octokit" to add a single file to your gitub repo. Adapted from a Ruby guide by http://mattgreensmith.net

addFileToGit.js

const Octokit = require('@octokit/rest'); // https://octokit.github.io/rest.js/

// Customize this stuff:
const auth = 'your-key-generated-in-github-ui'; // PRIVATE!
const owner = 'repo-owner';
const repo = 'your-repo-name';

// Constants
const userAgent = 'git commiter v1';
const ref = 'heads/master';
const FILE = '100644'; // commit mode

const octokit = new Octokit({ auth, userAgent });

// Create a single-file commit on top of head.
async function addFileToGit(path, content) {

  // 1. Get the sha of the last commit
  const { data: { object } } = await octokit.git.getRef({repo, owner, ref});
  const sha_latest_commit = object.sha;

  // 2. Find and store the SHA for the tree object that the heads/master commit points to.
  const { data: { tree }} = await octokit.git.getCommit({repo, owner, commit_sha: sha_latest_commit})
  const sha_base_tree = tree.sha;

  // 3. Create some content
  const { data: { sha: blob_sha } } = await octokit.git.createBlob({
    repo,
    owner,
    encoding: "utf-8",
    content,
  });

  // 4. Create a new tree with the content in place
  const { data: new_tree } = await octokit.git.createTree({
    repo,
    owner,
    base_tree: sha_base_tree, // if we don't set this, all other files will show up as deleted.
    tree: [
      {
        path,
        mode: FILE,
        type: 'blob',
        sha: blob_sha,
      }
    ],
  });

  // 5. Create a new commit with this tree object
  const { data: new_commit } = await octokit.git.createCommit({
    repo,
    owner,
    message: "addFileToGit.js scripted commit",
    tree: new_tree.sha,
    parents: [
      sha_latest_commit
    ],
  });

  // 6. Move the reference to point at new commit.
  const { data: { object: updated_ref } } = await octokit.git.updateRef({
    repo,
    owner,
    ref,
    sha: new_commit.sha, 
  });
}

This function makes it easy to write a string as a file to a git repo.