Temporary hack to fix terser output (#5461)

Author: hsubox76

.changeset/spicy-moose-approve.md

@@ -0,0 +1,5 @@
+---
+'@firebase/firestore': patch
+---
+
+Temporary fix for a bug causing `initializeFirestore()` to not work with certain bundling pipelines.

packages/firestore/rollup.shared.js

@@ -145,6 +145,13 @@ const manglePrivatePropertiesOptions = {
     beautify: true
   },
   mangle: {
+    // Temporary hack fix for an issue where mangled code causes some downstream
+    // bundlers (Babel?) to confuse the same variable name in different scopes.
+    // This can be removed if the problem in the downstream library is fixed
+    // or if terser's mangler provides an option to avoid mangling everything
+    // that isn't a property.
+    // See issue: https://github.com/firebase/firebase-js-sdk/issues/5384
+    reserved: ['_getProvider'],
     properties: {
       regex: /^__PRIVATE_/,
       // All JS Keywords are reserved. Although this should be taken cared of by
@@ -284,7 +291,12 @@ exports.es2017ToEs5Plugins = function (mangled = false) {
           comments: 'all',
           beautify: true
         },
-        mangle: true
+        // See comment above `manglePrivatePropertiesOptions`. This build did
+        // not have the identical variable name issue but we should be
+        // consistent.
+        mangle: {
+          reserved: ['_getProvider']
+        }
       }),
       sourcemaps()
     ];