nodegit-write.js

{
  //given:
  const repo - bare Repository {}
  const branchCommit = repo.getBranchCommit(branchName),
  
  // actual method: 
  open: async (fn) => {
        const tree = await branchCommit.getTree();
        const treeBuilder = await nodegit.Treebuilder.create(repo, tree);

        const write = async (fileName, body) => {
          const buffer = Buffer.from(body, 'UTF-8');
          const oid = await nodegit.Blob.createFromBuffer(repo, buffer, buffer.length);
          await treeBuilder.insert(fileName, oid, 33188);
        };

        await fn({ write });

        const id = await treeBuilder.write();
        return id;
      },
      //..
       
}