How to use the isomorphic-git.checkout function in isomorphic-git

private async _checkout(branch: string) {
        this._index.splice(0, this._index.length);
        await checkout({
            dir: this.projectDir,
            ref: branch
        });
    }

async checkout() {
    log(`Attempting to checkout at ${this.finalPath}`);
    await mkdir(this.finalPath);
    await git.checkout({
      gitdir: this.clonePath,
      dir: this.finalPath,
      ref: this.repo.sha
    });
  }

export async function checkoutBranch(
  projectRoot: string,
  branchName: string
): Promise {
  return git.checkout({ dir: projectRoot, ref: branchName })
}

export async function checkoutBranch(
  projectRoot: string,
  branchName: string
): Promise {
  return git.checkout({ fs, dir: projectRoot, ref: branchName })
}