Allow single string to/cc/bcc fields

Author: Ehesp

firestore-send-email/POSTINSTALL.md

@@ -4,7 +4,20 @@ To test out this extension, add a document with a `to` field and a `message` fie
 
 ```
 admin.firestore().collection('${param:MAIL_COLLECTION}').add({
-  to: ['someone@example.com'],
+  to: 'someone@example.com',
+  message: {
+    subject: 'Hello from Firebase!',
+    text: 'This is the plaintext section of the email body.',
+    html: 'This is the <code>HTML</code> section of the email body.',
+  }
+}).then(() => console.log('Queued email for delivery!'));
+```
+
+To send an email to multiple email addresses at once, provide an array to the `to` field:
+
+```
+admin.firestore().collection('${param:MAIL_COLLECTION}').add({
+  to: ['someone@example.com', 'someone-else@example.com'],
   message: {
     subject: 'Hello from Firebase!',
     text: 'This is the plaintext section of the email body.',
@@ -22,12 +35,12 @@ After its installation, this extension monitors all document writes to the `${pa
 The top-level fields of the document supply the email sender and recipient information. Available fields are:
 
 * **from:** The sender's email address. If not specified in the document, uses the configured "Default FROM address" parameter.
-* **replyTo:** The reply-to email address. If not specified in the document, uses the configured "Default REPLY-TO address" parameter.
-* **to:** An array containing the recipient email addresses.
+* **replyTo:** The reply-to email address. If not specified in the document, uses the required configured "Default REPLY-TO address" parameter.
+* **to:** An email address or an array containing the recipient email addresses.
 * **toUids:** An array containing the recipient UIDs.
-* **cc:** An array containing the CC recipient email addresses.
+* **cc:** An email address or an array containing the CC recipient email addresses.
 * **ccUids:** An array containing the CC recipient UIDs.
-* **bcc:** An array containing the BCC recipient email addresses.
+* **bcc:** An email address or an array containing the BCC recipient email addresses.
 * **bccUids:** An array containing the BCC recipient UIDs.
 
 **NOTE:** The `toUids`, `ccUids`, and `bccUids` options deliver emails based on user UIDs keyed to email addresses within a Cloud Firestore document. To use these recipient options, you need to specify a Cloud Firestore collection for the extension's "Users collection" parameter. The extension can then read the `email` field for each UID specified in the `toUids`, `ccUids`, and/or `bccUids` fields.

firestore-send-email/PREINSTALL.md

@@ -6,7 +6,7 @@ Here's a basic example document write that would trigger this extension:
 
 ```js
 admin.firestore().collection('mail').add({
-  to: ['someone@example.com'],
+  to: 'someone@example.com',
   message: {
     subject: 'Hello from Firebase!',
     html: 'This is an <code>HTML</code> email body.',

firestore-send-email/functions/src/index.ts

@@ -107,19 +107,25 @@ async function preparePayload(payload: QueuePayload): Promise<QueuePayload> {
   let cc: string[] = [];
   let bcc: string[] = [];
 
-  if (payload.to) {
+  if (typeof payload.to === 'string') {
+    to = [payload.to];
+  } else if (payload.to) {
     validateFieldArray("to", payload.to);
-    to = [...to, ...payload.to];
+    to = to.concat(payload.to);
   }
 
-  if (payload.cc) {
+  if (typeof payload.cc === 'string') {
+    cc = [payload.cc];
+  } else if (payload.cc) {
     validateFieldArray("cc", payload.cc);
-    cc = [...cc, ...payload.cc];
+    cc = cc.concat(payload.cc);
   }
 
-  if (payload.bcc) {
+  if (typeof payload.bcc === 'string') {
+    bcc = [payload.bcc];
+  } else if (payload.bcc) {
     validateFieldArray("bcc", payload.bcc);
-    bcc = [...bcc, ...payload.bcc];
+    bcc = bcc.concat(payload.bcc);
   }
 
   if (!payload.toUids && !payload.ccUids && !payload.bccUids) {