Fix instanceof FileAttachment.

Author: mbostock

src/fileAttachment.js

@@ -48,9 +48,12 @@ export function NoFileAttachments(name) {
 }
 
 export default function FileAttachments(resolve) {
-  return name => {
-    const url = resolve(name += ""); // Returns a Promise, string, or null.
-    if (url == null) throw new Error(`File not found: ${name}`);
-    return new FileAttachment(url, name);
-  };
+  return Object.assign(
+    name => {
+      const url = resolve(name += ""); // Returns a Promise, string, or null.
+      if (url == null) throw new Error(`File not found: ${name}`);
+      return new FileAttachment(url, name);
+    },
+    {prototype: FileAttachment.prototype} // instanceof
+  );
 }

test/fileAttachments-test.js

@@ -8,20 +8,42 @@ test("FileAttachments is exported by stdlib", t => {
 });
 
 test("FileAttachment ensures that URLs are strings", async t => {
-  const fileAttachments = FileAttachments((name) =>
+  const FileAttachment = FileAttachments((name) =>
     new URL(`https://example.com/${name}.js`)
   );
-  const file = fileAttachments("filename");
+  const file = FileAttachment("filename");
   t.equal(file.constructor.name, "FileAttachment");
   t.equal(await file.url(), "https://example.com/filename.js");
   t.end();
 });
 
+test("FileAttachment returns instances of FileAttachment", async t => {
+  const FileAttachment = FileAttachments((name) =>
+    new URL(`https://example.com/${name}.js`)
+  );
+  const file = FileAttachment("filename");
+  t.true(file instanceof FileAttachment);
+  t.end();
+});
+
+test("FileAttachment cannot be used as a constructor", async t => {
+  const FileAttachment = FileAttachments((name) =>
+    new URL(`https://example.com/${name}.js`)
+  );
+  try {
+    new FileAttachment("filename");
+    t.fail();
+  } catch (error) {
+    t.equal(error.message, "FileAttachment is not a constructor");
+  }
+  t.end();
+});
+
 test("FileAttachment works with Promises that resolve to URLs", async t => {
-  const fileAttachments = FileAttachments(async (name) =>
+  const FileAttachment = FileAttachments(async (name) =>
     new URL(`https://example.com/${name}.js`)
   );
-  const file = fileAttachments("otherfile");
+  const file = FileAttachment("otherfile");
   t.equal(file.constructor.name, "FileAttachment");
   t.equal(await file.url(), "https://example.com/otherfile.js");
   t.end();