How to deploy subfolder to Github Pages

We can use git subtree to do this. And also nest git commands to execute force push (Reference).

git push origin `git subtree split --prefix subfolder-name-here master`:gh-pages --force

Put this command to package.json file like bellow.

{
    "name": "My App",
    "dependencies": { ... },
    "scripts": {
        "deploy": "git push origin `git subtree split --prefix subfolder-name-here master`:gh-pages --force"
    }
}

After that, we can use npm run deploy command to deploy subfolder to gh-pages branch.