First.

Author: jamesdaniels

packages/util/defaults.d.ts

@@ -0,0 +1,8 @@
+import { FirebaseOptions } from "@firebase/app-types";
+
+declare const FirebaseDefaults: {
+    config?: FirebaseOptions;
+    emulatorHosts?: Record<string, string|undefined>;
+} | undefined;
+
+export default FirebaseDefaults;

packages/util/defaults.js

@@ -0,0 +1 @@
+module.exports = undefined

packages/util/defaults.mjs

@@ -0,0 +1 @@
+export default undefined

packages/util/package.json

@@ -21,10 +21,19 @@
       },
       "default": "./dist/index.esm2017.js"
     },
+    "./postinstall": {
+      "types": "./defaults.d.ts",
+      "require": "./defaults.js",
+      "import": "./defaults.mjs",
+      "default": "./defaults.mjs"
+    },
     "./package.json": "./package.json"
   },
   "files": [
-    "dist"
+    "dist",
+    "defaults.*js",
+    "defaults.d.ts",
+    "postinstall.mjs"
   ],
   "scripts": {
     "lint": "eslint -c .eslintrc.js '**/*.ts' --ignore-path '../../.gitignore'",
@@ -40,7 +49,8 @@
     "test:node": "TS_NODE_CACHE=NO TS_NODE_COMPILER_OPTIONS='{\"module\":\"commonjs\"}' nyc --reporter lcovonly -- mocha test/**/*.test.* --config ../../config/mocharc.node.js",
     "trusted-type-check": "tsec -p tsconfig.json --noEmit",
     "api-report": "api-extractor run --local --verbose",
-    "typings:public": "node ../../scripts/build/use_typings.js ./dist/util-public.d.ts"
+    "typings:public": "node ../../scripts/build/use_typings.js ./dist/util-public.d.ts",
+    "postinstall": "node ./postinstall.mjs"
   },
   "license": "Apache-2.0",
   "dependencies": {

packages/util/postinstall.mjs

@@ -0,0 +1,41 @@
+import { writeFile } from "fs/promises";
+
+let firebaseConfig = {};
+if (process.env.FIREBASE_CONFIG?.startsWith("{")) {
+    // TODO probably want a more robust yaml parse
+    firebaseConfig = Object.fromEntries(process.env.FIREBASE_CONFIG.match(/[^(\:\{\},)]+\:[^(,})]+/g).map(it => {
+        const parts = it.split(":");
+        return [parts[0], parts.slice(1).join(":")]
+    }));
+}
+
+const projectId = firebaseConfig.projectId;
+const appId = firebaseConfig.appId;
+const apiKey = firebaseConfig.apiKey;
+
+const config = projectId && appId && apiKey && await (await fetch(
+    `https://firebase.googleapis.com/v1alpha/projects/${projectId}/apps/${appId}/webConfig`,
+    { headers: { "x-goog-api-key": apiKey } }
+)).json();
+
+if (config) {
+    config.apiKey = apiKey;
+}
+
+let emulatorHosts = {
+    firestore: process.env.FIRESTORE_EMULATOR_HOST,
+    database: process.env.FIREBASE_DATABASE_EMULATOR_HOST,
+    storage: process.env.FIREBASE_STORAGE_EMULATOR_HOST,
+    auth: process.env.FIREBASE_AUTH_EMULATOR_HOST,
+};
+
+if (!Object.values(emulatorHosts).filter(it => it).length) {
+    emulatorHosts = undefined;
+}
+
+const defaults = (config || emulatorHosts) && { config, emulatorHosts };
+
+await Promise.all([
+    writeFile("./defaults.js", `module.exports = ${JSON.stringify(defaults)}`),
+    writeFile("./defaults.mjs", `export default ${JSON.stringify(defaults)}`),
+]);

packages/util/src/defaults.ts

@@ -17,6 +17,8 @@
 
 import { base64Decode } from './crypt';
 import { getGlobal } from './global';
+// @ts-ignore
+import postinstallDefaults from "@firebase/util/postinstall";
 
 /**
  * Keys for experimental properties on the `FirebaseDefaults` object.
@@ -100,6 +102,7 @@ const getDefaultsFromCookie = (): FirebaseDefaults | undefined => {
 export const getDefaults = (): FirebaseDefaults | undefined => {
   try {
     return (
+      postinstallDefaults ||
       getDefaultsFromGlobal() ||
       getDefaultsFromEnvVariable() ||
       getDefaultsFromCookie()