Ensure that FileAttachment URLs are strings, for compatibility

Author: jashkenas

src/fileAttachment.js

@@ -7,7 +7,7 @@ async function remote_fetch(file) {
 class FileAttachment {
   constructor(url, name) {
     Object.defineProperties(this, {
-      _url: {value: url},
+      _url: {value: url + ""},
       name: {value: name, enumerable: true}
     });
   }

test/fileAttachments-test.js

@@ -6,3 +6,13 @@ test("FileAttachments is exported by stdlib", t => {
   t.equal(FileAttachments.name, "FileAttachments");
   t.end();
 });
+
+test("FileAttachment ensures that URLs are strings", async t => {
+  const fileAttachments = FileAttachments((name) =>
+    new URL(`https://example.com/${name}.js`)
+  );
+  const file = fileAttachments("filename");
+  t.equal(file.constructor.name, "FileAttachment");
+  t.equal(await file.url(), "https://example.com/filename.js");
+  t.end();
+});