Scripting: Document the .mjs treatment and fixed some issues

* Make __filename also available in .mjs files
* Fixed reporting of errors that happen during importing of modules

docs/reference/scripting.rst

@@ -83,6 +83,16 @@ An extension can be placed either directly in an extensions directory, or in a
 sub-directory. All scripts files found in these directories are executed on
 startup.
 
+.. raw:: html
+
+   <div class="new">Since Tiled 1.8</div>
+
+When using the ``.mjs`` extension, script files are loaded as `JavaScript
+modules`_. They will then be able to use the `import`_ and `export`_ statements
+to split up their functionality over multiple JavaScript files. Such extensions
+also don't pollute the global scope, avoiding potential name collisions between
+different extensions.
+
 When any loaded script is changed or when any files are added/removed from the
 extensions directory, the script engine is automatically reinstantiated and the
 scripts are reloaded. This way there is no need to restart Tiled when
@@ -121,3 +131,6 @@ __filename
 .. _List of JavaScript Objects and Functions: https://doc.qt.io/qt-5/qtqml-javascript-functionlist.html
 .. _QML module: https://doc.qt.io/qt-5/qtqml-index.html
 .. _@mapeditor/tiled-api: https://www.npmjs.com/package/@mapeditor/tiled-api
+.. _JavaScript modules: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules
+.. _import: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import
+.. _export: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export

src/tiled/scriptmanager.cpp

@@ -258,7 +258,27 @@ void ScriptManager::loadExtension(const QString &path)
             }
 #else
             Tiled::INFO(tr("Importing module '%1'").arg(absolutePath));
-            checkError(mEngine->importModule(absolutePath));
+
+            QJSValue globalObject = mEngine->globalObject();
+            globalObject.setProperty(QStringLiteral("__filename"), absolutePath);
+
+            QJSValue result = mEngine->importModule(absolutePath);
+
+            // According to the documentation, importModule could return an
+            // error object, though in practice this doesn't appear to happen.
+            if (!checkError(result)) {
+#if QT_VERSION >= QT_VERSION_CHECK(6, 1, 0)
+                // This appears to be the way to report exceptions
+                checkError(mEngine->catchError());
+#else
+                // With Qt 5 it seems we need to get a little creative, like
+                // calling evaluate to let that catch a potentially raised
+                // exception.
+                checkError(mEngine->evaluate(QString()));
+#endif
+            }
+
+            globalObject.deleteProperty(QStringLiteral("__filename"));
 #endif
         }
         mWatcher.addPath(absolutePath);