background.js

chrome.runtime.onInstalled.addListener(() => {
  chrome.contextMenus.create({
    id: 'redirectMenuItem',
    title: 'Open in JSCAD',
    contexts: ['link'],
  })
})

chrome.action.onClicked.addListener(tab => {
  chrome.tabs.query({active: true, currentWindow: true}, tabs => {
    chrome.scripting.executeScript({
      target: {tabId: tab.id},
      function: redirect,
      args: [tabs[0].url],
    })
  })
})

chrome.contextMenus.onClicked.addListener((info, tab) => {
  if (info.menuItemId === 'redirectMenuItem')
    chrome.scripting.executeScript({
      target: {tabId: tab.id},
      function: redirect,
      args: [info.linkUrl],
    })
})

async function redirect(link) {
  let url = new URL(link)
  if (!url.pathname.endsWith('.js')) return alert('Not a JavaScript link')
  if (url.host == 'github.com') url = (await fetch(url + '?raw=true')).url
  location.assign('https://jscad.app#' + url)
}