[lib] sendMessage.multicast supports batching when >= 500 ids are given

src/lib/sendMessage.js

@@ -10,16 +10,41 @@ const notify = async (token, message) => {
   lineNotify(token, { message: message });
 };
 
+/**
+ * Split large array into list of batches, with max size being batchSize for each batch
+ * @param {Array<*>} array
+ * @param {number} batchSize
+ * @returns {Array<Array<*>>}
+ */
+function batch(array, batchSize) {
+  return array.reduce(
+    (batches, item) => {
+      if (batches[batches.length - 1].length >= batchSize) {
+        batches.push([]);
+      }
+      batches[batches.length - 1].push(item);
+      return batches;
+    },
+    [[]]
+  );
+}
+
 /**
  * https://developers.line.biz/en/reference/messaging-api/#send-multicast-message
  * @param {string[]} userIds
  * @param {object[]} messages - Array of line message objects, max size:5
  */
-const multicast = (userIds, messages) =>
-  lineClient.post('/message/multicast', {
-    to: userIds,
-    messages: messages,
-  });
+const multicast = async (userIds, messages) => {
+  for (const userIdBatch of batch(
+    userIds,
+    500 /* Multicast can send to 500 ppl in max each time */
+  )) {
+    await lineClient.post('/message/multicast', {
+      to: userIdBatch,
+      messages: messages,
+    });
+  }
+};
 
 /**
  * https://developers.line.biz/en/reference/messaging-api/#send-push-message