Move URL value coercion to the .url() function, support asynchronously resolved URLs.

Author: jashkenas

src/fileAttachment.js

@@ -7,12 +7,12 @@ async function remote_fetch(file) {
 class FileAttachment {
   constructor(url, name) {
     Object.defineProperties(this, {
-      _url: {value: url + ""},
+      _url: {value: url},
       name: {value: name, enumerable: true}
     });
   }
   async url() {
-    return this._url;
+    return (await this._url) + "";
   }
   async blob() {
     return (await remote_fetch(this)).blob();

test/fileAttachments-test.js

@@ -16,3 +16,13 @@ test("FileAttachment ensures that URLs are strings", async t => {
   t.equal(await file.url(), "https://example.com/filename.js");
   t.end();
 });
+
+test("FileAttachment works with Promises that resolve to URLs", async t => {
+  const fileAttachments = FileAttachments((name) =>
+    new Promise((resolve) => resolve(new URL(`https://example.com/${name}.js`)))
+  );
+  const file = fileAttachments("otherfile");
+  t.equal(file.constructor.name, "FileAttachment");
+  t.equal(await file.url(), "https://example.com/otherfile.js");
+  t.end();
+});