You don't have to use an npm script to do things. You can do it with git commands. npm script is an automation to make things faster.
Re: netlify
GitHub Pages is completely free but has limitations if the traffic bandwidth becomes really really high.
Netlify has a free tier but has limitation on the build minutes in the free tier.
You can choose either of these or a different static hosting service to deploy your site. It's upto you.
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Re: script
You don't have to use an npm script to do things. You can do it with git commands. npm script is an automation to make things faster.
Re: netlify
GitHub Pages is completely free but has limitations if the traffic bandwidth becomes really really high.
Netlify has a free tier but has limitation on the build minutes in the free tier.
You can choose either of these or a different static hosting service to deploy your site. It's upto you.