diff --git a/service_worker.js b/service_worker.js
index 704f51c..6867862 100644
--- a/service_worker.js
+++ b/service_worker.js
@@ -13,18 +13,51 @@ self.addEventListener('activate', function(event) {
 });
 */
 
-let data
+let dir_handle
 
 self.addEventListener('message', async function(e) {
   const msg = e.data
   let reply
   if(msg.type == 'SET') {
-    data = msg.data
+    dir_handle = msg.data
     reply = null
   } else if(msg.type == 'GET') {
-    reply = data
+    reply = dir_handle
   } else {
     throw new Error('unknown message type: ' + msg.type)
   }
   e.ports[0].postMessage(reply)
 })
+
+// Fake URL base prepended by code responsible for module loading
+const FAKE_URL_BASE = 'https://leporello.import/'
+
+self.addEventListener("fetch", event => {
+  if(event.request.url.startsWith(FAKE_URL_BASE)) {
+    if(dir_handle != null) {
+      const headers = new Headers([
+        ['Content-Type', 'text/javascript']
+      ])
+      const path = event.request.url.replace(FAKE_URL_BASE, '')
+      const response = read_file(dir_handle, path).then(file => 
+        new Response(file, {headers})
+      )
+      event.respondWith(response)
+    }
+  }
+})
+
+const read_file = async (dir_handle, filename) => {
+  if(typeof(filename) == 'string') {
+    filename = filename.split('/')
+  }
+  const [first, ...rest] = filename
+  if(rest.length == 0) {
+    const fhandle = await dir_handle.getFileHandle(first)
+    const file_data = await fhandle.getFile()
+    return await file_data.text()
+  } else {
+    const nested_dir_handle = await dir_handle.getDirectoryHandle(first)
+    return read_file(nested_dir_handle, rest)
+  }
+}
diff --git a/src/effects.js b/src/effects.js
index 58b10d0..82fd12f 100644
--- a/src/effects.js
+++ b/src/effects.js
@@ -10,7 +10,14 @@ const load_external_imports = async state => {
   }
   const urls = state.loading_external_imports_state.external_imports
   const results = await Promise.allSettled(
-    urls.map(u => import(u))
+    urls.map(u => import(
+      /^\w+:\/\//.test(u)    
+        ? // starts with protocol, import as is
+          u
+        : //local path, load using File System Access API, see service_worker.js
+          // Append fake host that will be intercepted in service worker
+          'https://leporello.import/' + u
+    ))
   )
   const modules = Object.fromEntries(
     results.map((r, i) => (
diff --git a/src/find_definitions.js b/src/find_definitions.js
index 06f7135..86f9521 100644
--- a/src/find_definitions.js
+++ b/src/find_definitions.js
@@ -154,39 +154,6 @@ export const find_definitions = (ast, scope = {}, closure_scope = {}, module_nam
   }
 }
 
-// see https://stackoverflow.com/a/29855511
-
-// Joins path segments.  Preserves initial "/" and resolves ".." and "."
-// Does not support using ".." to go above/outside the root.
-// This means that join("foo", "../../bar") will not resolve to "../bar"
-
-const join = new Function(`
-  // Split the inputs into a list of path commands.
-  var parts = [];
-  for (var i = 0, l = arguments.length; i < l; i++) {
-    parts = parts.concat(arguments[i].split("/"));
-  }
-  // Interpret the path commands to get the new resolved path.
-  var newParts = [];
-  for (i = 0, l = parts.length; i < l; i++) {
-    var part = parts[i];
-    // Remove leading and trailing slashes
-    // Also remove "." segments
-    if (!part || part === ".") continue;
-    // Interpret ".." to pop the last segment
-    if (part === "..") newParts.pop();
-    // Push new path segments.
-    else newParts.push(part);
-  }
-  // Preserve the initial slash if there was one.
-  if (parts[0] === "") newParts.unshift("");
-  // Turn back into a single string path.
-  return newParts.join("/") || (newParts.length ? "/" : ".");
-`)
-
-const concat_path = (base,  imported) => 
-  base == '' ? join(imported) : join(base, '..', imported)
-
 export const find_export = (name, module) => {
   return map_find(module.stmts, n => {
     if(n.type != 'export') {
@@ -197,6 +164,16 @@ export const find_export = (name, module) => {
   })
 }
 
+const BASE = 'dummy://dummy/'
+const concat_path = (base, i) => {
+  const result = new URL(i, BASE + base).toString()
+  if(result.lastIndexOf(BASE) == 0) {
+    return result.replace(BASE, '')
+  } else {
+    return result
+  }
+  return result
+}
 
 export const topsort_modules = (modules) => {
   const sort_module_deps = (module) => {
diff --git a/src/globals.js b/src/globals.js
index 19e36df..a2fea60 100644
--- a/src/globals.js
+++ b/src/globals.js
@@ -2,6 +2,7 @@ export const globals = new Set([
   'globalThis',
   // TODO Promise,
   // TODO Symbol
+  'URL',
   'Set', 
   'Map', 
   "Infinity",
diff --git a/test/test.js b/test/test.js
index ef67200..0ee167d 100644
--- a/test/test.js
+++ b/test/test.js
@@ -2310,7 +2310,7 @@ const y = x()`
       on_async_call: (calls) => {console.log('test on async call', calls)}
     })
     globalThis.__run_async_call()
-    delete globalThis.__run_async_call
+    //delete globalThis.__run_async_call
   }),
 
 ]