diff --git a/app/utils/reaction.js b/app/utils/reaction.js
index 984764b1c..fe091688f 100644
--- a/app/utils/reaction.js
+++ b/app/utils/reaction.js
@@ -13,7 +13,7 @@ export function generateUserProfilesById(userProfiles = []) {
 
 export function getMissingUserIds(userProfilesById = {}, allUserIds = []) {
     return allUserIds.reduce((acc, userId) => {
-        if (userProfilesById[userId]) {
+        if (!userProfilesById[userId]) {
             acc.push(userId);
         }
 
diff --git a/app/utils/reaction.test.js b/app/utils/reaction.test.js
new file mode 100644
index 000000000..1c0d1d392
--- /dev/null
+++ b/app/utils/reaction.test.js
@@ -0,0 +1,176 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import {
+    getMissingUserIds,
+    getReactionsByName,
+    getSortedReactionsForHeader,
+    getUniqueUserIds,
+} from 'app/utils/reaction';
+
+import {ALL_EMOJIS} from 'app/constants/emoji';
+
+describe('getMissingUserIds', () => {
+    const testCases = [{
+        name: 'Null inputs',
+        output: [],
+    }, {
+        name: 'Should return correct missing user ID, last position',
+        userProfilesById: {user_id_1: {}},
+        allUserIds: ['user_id_1', 'user_id_2'],
+        output: ['user_id_2'],
+    }, {
+        name: 'Should return correct missing user ID, first position',
+        userProfilesById: {user_id_2: {}},
+        allUserIds: ['user_id_1', 'user_id_2'],
+        output: ['user_id_1'],
+    }, {
+        name: 'Should return correct missing user ID, mid position',
+        userProfilesById: {user_id_3: {}, user_id_1: {}},
+        allUserIds: ['user_id_1', 'user_id_2', 'user_id_3'],
+        output: ['user_id_2'],
+    }, {
+        name: 'Should return correct missing user ID, all',
+        userProfilesById: {},
+        allUserIds: ['user_id_1', 'user_id_2', 'user_id_3'],
+        output: ['user_id_1', 'user_id_2', 'user_id_3'],
+    }];
+
+    for (const testCase of testCases) {
+        it(testCase.name, () => {
+            expect(getMissingUserIds(testCase.userProfilesById, testCase.allUserIds)).toEqual(testCase.output);
+        });
+    }
+});
+
+describe('getReactionsByName', () => {
+    const testCases = [{
+        name: 'Null input',
+        output: {},
+    }, {
+        name: 'Should match reactions by name, single',
+        reactions: [{name: 'thumbs_up', emoji_name: '+1', post_id: 'post_id_1', user_id: 'user_id_1'}],
+        output: {'+1': [{emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_1'}]},
+    }, {
+        name: 'Should match reactions by name, many',
+        reactions: [
+            {name: 'thumbs_up', emoji_name: '+1', post_id: 'post_id_1', user_id: 'user_id_1'},
+            {name: 'thumbs_up', emoji_name: '+1', post_id: 'post_id_1', user_id: 'user_id_2'},
+        ],
+        output: {
+            '+1': [
+                {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_1'},
+                {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_2'},
+            ],
+        },
+    }, {
+        name: 'Should match reactions by name, many names',
+        reactions: [
+            {name: 'thumbs_up', emoji_name: '+1', post_id: 'post_id_1', user_id: 'user_id_1'},
+            {name: 'thumbs_up', emoji_name: '+1', post_id: 'post_id_1', user_id: 'user_id_2'},
+            {name: 'thumbs_down', emoji_name: '-1', post_id: 'post_id_1', user_id: 'user_id_1'},
+        ],
+        output: {
+            '+1': [
+                {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_1'},
+                {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_2'},
+            ],
+            '-1': [
+                {emoji_name: '-1', name: 'thumbs_down', post_id: 'post_id_1', user_id: 'user_id_1'},
+            ],
+        },
+    }];
+
+    for (const testCase of testCases) {
+        it(testCase.name, () => {
+            expect(getReactionsByName(testCase.reactions)).toEqual(testCase.output);
+        });
+    }
+});
+
+describe('getSortedReactionsForHeader', () => {
+    const testCases = [{
+        name: 'Null input',
+        output: [{count: 0, name: ALL_EMOJIS}],
+    }, {
+        name: 'Should match reactions for header, single',
+        reactionsByName: {'+1': [{emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_1'}]},
+        output: [
+            {count: 1, name: ALL_EMOJIS},
+            {
+                count: 1,
+                name: '+1',
+                reactions: [
+                    {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_1'},
+                ],
+            },
+        ],
+    }, {
+        name: 'Should match reactions for header, multiple',
+        reactionsByName: {
+            '+1': [
+                {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_1'},
+                {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_2'},
+            ],
+            '-1': [
+                {emoji_name: '-1', name: 'thumbs_down', post_id: 'post_id_1', user_id: 'user_id_1'},
+            ],
+        },
+        output: [
+            {count: 3, name: ALL_EMOJIS},
+            {
+                count: 2,
+                name: '+1',
+                reactions: [
+                    {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_1'},
+                    {emoji_name: '+1', name: 'thumbs_up', post_id: 'post_id_1', user_id: 'user_id_2'},
+                ],
+            },
+            {
+                count: 1,
+                name: '-1',
+                reactions: [
+                    {emoji_name: '-1', name: 'thumbs_down', post_id: 'post_id_1', user_id: 'user_id_1'},
+                ],
+            },
+        ],
+    }];
+
+    for (const testCase of testCases) {
+        it(testCase.name, () => {
+            expect(getSortedReactionsForHeader(testCase.reactionsByName)).toEqual(testCase.output);
+        });
+    }
+});
+
+describe('getUniqueUserIds', () => {
+    const testCases = [{
+        name: 'Null input',
+        output: [],
+    }, {
+        name: 'Should match unique user ID',
+        reactions: [{name: 'thumbs_up', emoji_name: '+1', post_id: 'post_id_1', user_id: 'user_id_1'}],
+        output: ['user_id_1'],
+    }, {
+        name: 'Should match unique user IDs',
+        reactions: [
+            {name: 'thumbs_up', emoji_name: '+1', post_id: 'post_id_1', user_id: 'user_id_1'},
+            {name: 'thumbs_down', emoji_name: '-1', post_id: 'post_id_1', user_id: 'user_id_1'},
+        ],
+        output: ['user_id_1'],
+    }, {
+        name: 'Should match unique user IDs',
+        reactions: [
+            {name: 'smile', emoji_name: 'smile', post_id: 'post_id_1', user_id: 'user_id_2'},
+            {name: 'thumbs_up', emoji_name: '+1', post_id: 'post_id_1', user_id: 'user_id_1'},
+            {name: 'thumbs_down', emoji_name: '-1', post_id: 'post_id_1', user_id: 'user_id_1'},
+        ],
+        output: ['user_id_2', 'user_id_1'],
+    }];
+
+    for (const testCase of testCases) {
+        it(testCase.name, () => {
+            expect(getUniqueUserIds(testCase.reactions)).toEqual(testCase.output);
+        });
+    }
+});