build.js

let httpPlugin = {
    name: 'http',
    setup(build) {
      let https = require('https')
      let http = require('http')
  
      // Intercept import paths starting with "http:" and "https:" so
      // esbuild doesn't attempt to map them to a file system location.
      // Tag them with the "http-url" namespace to associate them with
      // this plugin.
      build.onResolve({ filter: /^https?:\/\// }, args => ({
        path: args.path,
        namespace: 'http-url',
      }))
  
      // We also want to intercept all import paths inside downloaded
      // files and resolve them against the original URL. All of these
      // files will be in the "http-url" namespace. Make sure to keep
      // the newly resolved URL in the "http-url" namespace so imports
      // inside it will also be resolved as URLs recursively.
      build.onResolve({ filter: /.*/, namespace: 'http-url' }, args => ({
        path: new URL(args.path, args.importer).toString(),
        namespace: 'http-url',
      }))
  
      // When a URL is loaded, we want to actually download the content
      // from the internet. This has just enough logic to be able to
      // handle the example import from unpkg.com but in reality this
      // would probably need to be more complex.
      build.onLoad({ filter: /.*/, namespace: 'http-url' }, async (args) => {
        let contents = await new Promise((resolve, reject) => {
          function fetch(url) {
            console.log(`Downloading: ${url}`)
            let lib = url.startsWith('https') ? https : http
            let req = lib.get(url, res => {
              if ([301, 302, 307].includes(res.statusCode)) {
                fetch(new URL(res.headers.location, url).toString())
                req.abort()
              } else if (res.statusCode === 200) {
                let chunks = []
                res.on('data', chunk => chunks.push(chunk))
                res.on('end', () => resolve(Buffer.concat(chunks)))
              } else {
                reject(new Error(`GET ${url} failed: status ${res.statusCode}`))
              }
            }).on('error', reject)
          }
          fetch(args.path)
        })
        return { contents }
      })
    },
  }
  
  require('esbuild').build({
    entryPoints: ['worker.js'],
    bundle: true,
    outfile: 'index.js',
    plugins: [httpPlugin],
  }).catch(() => process.exit(1))