diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -97,19 +97,19 @@
 const _mapValuesWithKeys = _mapValues.convert({ cap: false });
 
 // Input must already be ordered!
-function threadsToMessageIDsFromMessageInfos(
+function mapThreadsToMessageIDsFromOrderedMessageInfos(
   orderedMessageInfos: $ReadOnlyArray<RawMessageInfo>,
 ): { [threadID: string]: string[] } {
-  const threads: { [threadID: string]: string[] } = {};
+  const threadsToMessageIDs: { [threadID: string]: string[] } = {};
   for (const messageInfo of orderedMessageInfos) {
     const key = messageID(messageInfo);
-    if (!threads[messageInfo.threadID]) {
-      threads[messageInfo.threadID] = [key];
+    if (!threadsToMessageIDs[messageInfo.threadID]) {
+      threadsToMessageIDs[messageInfo.threadID] = [key];
     } else {
-      threads[messageInfo.threadID].push(key);
+      threadsToMessageIDs[messageInfo.threadID].push(key);
     }
   }
-  return threads;
+  return threadsToMessageIDs;
 }
 
 function isThreadWatched(
@@ -144,7 +144,7 @@
     payload: { id: messageID(messageInfo), messageInfo },
   }));
 
-  const threadsToMessageIDs = threadsToMessageIDsFromMessageInfos(
+  const threadsToMessageIDs = mapThreadsToMessageIDsFromOrderedMessageInfos(
     orderedMessageInfos,
   );
   const lastPruned = Date.now();
@@ -406,7 +406,7 @@
   )(unshimmedNewMessagesOfWatchedThreads);
 
   const newMessageOps: MessageStoreOperation[] = [];
-  const threadsToMessageIDs = threadsToMessageIDsFromMessageInfos(
+  const threadsToMessageIDs = mapThreadsToMessageIDsFromOrderedMessageInfos(
     orderedNewMessageInfos,
   );
   const oldMessageInfosToCombine = [];