add environment variable to point admin sdk at rtdb emulator

src/auth/credential.ts

@@ -419,6 +419,23 @@ export class ApplicationDefaultCredential implements FirebaseCredential {
   }
 }
 
+export class DatabaseEmulatorCredential implements FirebaseCredential {
+  public getAccessToken(): Promise<GoogleOAuthAccessToken> {
+    return Promise.resolve({
+      expires_in: 1000000,
+      access_token: 'owner',
+    });
+  }
+
+  public getCertificate(): Certificate {
+    return {
+      clientEmail: '',
+      projectId: '',
+      privateKey: '',
+    };
+  }
+}
+
 function credentialFromFile(filePath: string, httpAgent?: Agent): Credential {
   const credentialsFile = readCredentialFile(filePath);
   if (typeof credentialsFile !== 'object') {

src/firebase-namespace.ts

@@ -24,6 +24,7 @@ import {
   Credential,
   CertCredential,
   RefreshTokenCredential,
+  DatabaseEmulatorCredential,
   ApplicationDefaultCredential,
 } from './auth/credential';
 
@@ -46,6 +47,13 @@ const DEFAULT_APP_NAME = '[DEFAULT]';
  */
 export const FIREBASE_CONFIG_VAR: string = 'FIREBASE_CONFIG';
 
+/**
+ * Constant holding the fully-qualified domain URI for a database emulator
+ * instance. If specified, the contents of this variable will be used to
+ * set `databaseURL` in FirebaseAppOptions.
+ */
+const FIREBASE_DATABASE_EMULATOR_HOST_VAR: string = 'FIREBASE_DATABASE_EMULATOR_HOST';
+
 
 let globalAppDefaultCred: ApplicationDefaultCredential;
 const globalCertCreds: { [key: string]: CertCredential } = {};
@@ -84,7 +92,9 @@ export class FirebaseNamespaceInternals {
   public initializeApp(options?: FirebaseAppOptions, appName = DEFAULT_APP_NAME): FirebaseApp {
     if (typeof options === 'undefined') {
       options = this.loadOptionsFromEnvVar();
-      options.credential = new ApplicationDefaultCredential();
+      if (!('credential' in options)) {
+        options.credential = new ApplicationDefaultCredential();
+      }
     }
     if (typeof appName !== 'string' || appName === '') {
       throw new FirebaseAppError(
@@ -253,12 +263,24 @@ export class FirebaseNamespaceInternals {
    */
   private loadOptionsFromEnvVar(): FirebaseAppOptions {
     const config = process.env[FIREBASE_CONFIG_VAR];
+    const dbEmulatorHost = process.env[FIREBASE_DATABASE_EMULATOR_HOST_VAR];
     if (!validator.isNonEmptyString(config)) {
+      if (validator.isNonEmptyString(dbEmulatorHost)) {
+        return {
+          credential: new DatabaseEmulatorCredential(),
+          databaseURL: dbEmulatorHost,
+        };
+      }
       return {};
     }
     try {
       const contents = config.startsWith('{') ? config : fs.readFileSync(config, 'utf8');
-      return JSON.parse(contents) as FirebaseAppOptions;
+      const options = JSON.parse(contents) as FirebaseAppOptions;
+      if (validator.isNonEmptyString(dbEmulatorHost)) {
+        options.credential = new DatabaseEmulatorCredential();
+        options.databaseURL = `${dbEmulatorHost}` + (('projectId' in options) ? `?ns=${options.projectId}` : ``);
+      }
+      return options;
     } catch (error) {
       // Throw a nicely formed error message if the file contents cannot be parsed
       throw new FirebaseAppError(