errors: don't rekey on primitive type

If an error is thrown before a module is loaded, we attempt to cache
source map against error object, rather than module object. We
can't do this if the error is a primitive type

Fixes nodejs#38945

lib/internal/source_map/source_map_cache.js

@@ -175,7 +175,11 @@ function sourcesToAbsolute(baseURL, data) {
 // Move source map from garbage collected module to alternate key.
 function rekeySourceMap(cjsModuleInstance, newInstance) {
   const sourceMap = cjsSourceMapCache.get(cjsModuleInstance);
-  if (sourceMap) {
+  // If an exception occurs before a module finishes loading it will removed
+  // from the module cache and our WeakMap. To allow a source map to still be
+  // applied to the stack trace when this happens, we rekey on the thrown error.
+  // This is only possible if the error is an object and not a primitive type.
+  if (sourceMap && typeof newInstance === 'object') {
     cjsSourceMapCache.set(newInstance, sourceMap);
   }
 }

test/fixtures/source-map/throw-string-original.js

@@ -0,0 +1,8 @@
+/*
+ * comments dropped by uglify.
+ */
+function Hello() {
+  throw 'goodbye';
+}
+
+Hello();

test/parallel/test-source-map-enable.js

@@ -307,6 +307,23 @@ function nextdir() {
   assert.ok(sourceMap);
 }
 
+// Does not attempt to rekey source map on primitive type.
+{
+  const coverageDirectory = nextdir();
+  const output = spawnSync(process.execPath, [
+    '--enable-source-maps',
+    require.resolve('../fixtures/source-map/throw-string.js'),
+  ], { env: { ...process.env, NODE_V8_COVERAGE: coverageDirectory } });
+  const sourceMap = getSourceMapFromCache(
+    'throw-string.js',
+    coverageDirectory
+  );
+  // Original stack trace.
+  assert.match(output.stderr.toString(), /goodbye/);
+  // Source map should have been serialized.
+  assert.ok(sourceMap);
+}
+
 function getSourceMapFromCache(fixtureFile, coverageDirectory) {
   const jsonFiles = fs.readdirSync(coverageDirectory);
   for (const jsonFile of jsonFiles) {