[WIP] Remove SystemJS usage, in development mode, from the worker

*Please note:* Unfortunately this can't land until https://bugzilla.mozilla.org/show_bug.cgi?id=1247687 is fixed in Firefox. (The patch has, however, been successfully tested in Google Chrome 87.)

This removes the final piece of SystemJS usage from the PDF.js library, thus allowing a fair bit of clean-up, and we now use *only* native `import`/`export` statements everywhere in development mode.

.eslintrc

@@ -26,7 +26,6 @@
   "globals": {
     "PDFJSDev": false,
     "exports": false,
-    "SystemJS": false,
   },
 
   "rules": {

package.json

@@ -47,8 +47,6 @@
     "stylelint": "^13.12.0",
     "stylelint-config-prettier": "^8.0.2",
     "stylelint-prettier": "^1.2.0",
-    "systemjs": "^0.21.6",
-    "systemjs-plugin-babel": "^0.0.25",
     "terser": "^5.6.0",
     "through2": "^4.0.2",
     "ttest": "^3.0.0",

src/display/api.js

@@ -1753,11 +1753,10 @@ const PDFWorker = (function PDFWorkerClosure() {
       // Workers aren't supported in Node.js, force-disabling them there.
       isWorkerDisabled = true;
 
-      if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("LIB")) {
-        fallbackWorkerSrc = "../pdf.worker.js";
-      } else {
-        fallbackWorkerSrc = "./pdf.worker.js";
-      }
+      fallbackWorkerSrc =
+        typeof PDFJSDev !== "undefined" && PDFJSDev.test("LIB")
+          ? "../pdf.worker.js"
+          : "./pdf.worker.js";
     } else if (typeof document === "object" && "currentScript" in document) {
       const pdfjsFilePath = document.currentScript?.src;
       if (pdfjsFilePath) {
@@ -1783,13 +1782,11 @@ const PDFWorker = (function PDFWorkerClosure() {
   }
 
   function getMainThreadWorkerMessageHandler() {
-    let mainWorkerMessageHandler;
     try {
-      mainWorkerMessageHandler = globalThis.pdfjsWorker?.WorkerMessageHandler;
+      return globalThis.pdfjsWorker?.WorkerMessageHandler || null;
     } catch (ex) {
-      /* Ignore errors. */
+      return null; // Ignore errors.
     }
-    return mainWorkerMessageHandler || null;
   }
 
   // Loads worker code into main thread.
@@ -1807,7 +1804,7 @@ const PDFWorker = (function PDFWorkerClosure() {
         return mainWorkerMessageHandler;
       }
       if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")) {
-        const worker = await import("pdfjs/core/worker.js");
+        const worker = await import("pdfjs/pdf.worker.js");
         return worker.WorkerMessageHandler;
       }
       if (
@@ -1934,9 +1931,11 @@ const PDFWorker = (function PDFWorkerClosure() {
             );
           }
 
-          // Some versions of FF can't create a worker on localhost, see:
-          // https://bugzilla.mozilla.org/show_bug.cgi?id=683280
-          const worker = new Worker(workerSrc);
+          const worker =
+            (typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")) &&
+            !workerSrc.endsWith("/build/pdf.worker.js")
+              ? new Worker(workerSrc, { type: "module" })
+              : new Worker(workerSrc);
           const messageHandler = new MessageHandler("main", "worker", worker);
           const terminateEarly = () => {
             worker.removeEventListener("error", onWorkerError);

src/pdf.worker.js

@@ -16,8 +16,10 @@
 import { WorkerMessageHandler } from "./core/worker.js";
 
 /* eslint-disable-next-line no-unused-vars */
-const pdfjsVersion = PDFJSDev.eval("BUNDLE_VERSION");
+const pdfjsVersion =
+  typeof PDFJSDev !== "undefined" ? PDFJSDev.eval("BUNDLE_VERSION") : void 0;
 /* eslint-disable-next-line no-unused-vars */
-const pdfjsBuild = PDFJSDev.eval("BUNDLE_BUILD");
+const pdfjsBuild =
+  typeof PDFJSDev !== "undefined" ? PDFJSDev.eval("BUNDLE_BUILD") : void 0;
 
 export { WorkerMessageHandler };