diff --git a/src/actions/common/track.ts b/src/actions/common/track.ts
index cb3503b..5e3909d 100644
--- a/src/actions/common/track.ts
+++ b/src/actions/common/track.ts
@@ -6,13 +6,13 @@ import GeneralTrack from '@/shapes/model/graduation/GeneralTrack';
 import MajorTrack from '@/shapes/model/graduation/MajorTrack';
 import AdditionalTrack from '@/shapes/model/graduation/AdditionalTrack';
 
-interface TracksProps {
+export interface Tracks {
   general: GeneralTrack[];
   major: MajorTrack[];
   additional: AdditionalTrack[];
 }
 
-export function setTracks(tracks: TracksProps) {
+export function setTracks(tracks: Tracks) {
   return {
     type: SET_TRACKS,
     tracks: tracks,
diff --git a/src/reducers/common/index.js b/src/reducers/common/index.ts
similarity index 83%
rename from src/reducers/common/index.js
rename to src/reducers/common/index.ts
index a3cb827..76bf73f 100644
--- a/src/reducers/common/index.js
+++ b/src/reducers/common/index.ts
@@ -11,4 +11,5 @@ const CombinedReducer = combineReducers({
   media: media,
 });
 
+export type CommonState = ReturnType<typeof CombinedReducer>;
 export default CombinedReducer;
diff --git a/src/reducers/common/media.js b/src/reducers/common/media.js
deleted file mode 100644
index aecca3a..0000000
--- a/src/reducers/common/media.js
+++ /dev/null
@@ -1,18 +0,0 @@
-import { SET_IS_PORTRAIT } from '../../actions/common/media';
-
-const initialState = {
-  isPortrait: false,
-};
-
-export const reducer = (state = initialState, action) => {
-  switch (action.type) {
-    case SET_IS_PORTRAIT:
-      return Object.assign({}, state, {
-        isPortrait: action.isPortrait,
-      });
-    default:
-      return state;
-  }
-};
-
-export default reducer;
diff --git a/src/reducers/common/media.ts b/src/reducers/common/media.ts
new file mode 100644
index 0000000..f373bfc
--- /dev/null
+++ b/src/reducers/common/media.ts
@@ -0,0 +1,20 @@
+import { MediaAction, SET_IS_PORTRAIT } from '../../actions/common/media';
+
+interface MediaState {
+  isPortrait: boolean;
+}
+
+const initialState: MediaState = {
+  isPortrait: false,
+};
+
+const media = (state = initialState, action: MediaAction): MediaState => {
+  switch (action.type) {
+    case SET_IS_PORTRAIT:
+      return { ...state, isPortrait: action.isPortrait };
+    default:
+      return state;
+  }
+};
+
+export default media;
diff --git a/src/reducers/common/semester.js b/src/reducers/common/semester.js
deleted file mode 100644
index a4829b0..0000000
--- a/src/reducers/common/semester.js
+++ /dev/null
@@ -1,18 +0,0 @@
-import { SET_SEMESTERS } from '../../actions/common/semester';
-
-const initialState = {
-  semesters: null,
-};
-
-export const reducer = (state = initialState, action) => {
-  switch (action.type) {
-    case SET_SEMESTERS:
-      return Object.assign({}, state, {
-        semesters: action.semesters,
-      });
-    default:
-      return state;
-  }
-};
-
-export default reducer;
diff --git a/src/reducers/common/semester.ts b/src/reducers/common/semester.ts
new file mode 100644
index 0000000..537f42f
--- /dev/null
+++ b/src/reducers/common/semester.ts
@@ -0,0 +1,21 @@
+import { SET_SEMESTERS, SemesterAction } from '../../actions/common/semester';
+import Semester from '@/shapes/model/subject/Semester';
+
+interface SemesterState {
+  semesters: Semester[] | null;
+}
+
+const initialState: SemesterState = {
+  semesters: null,
+};
+
+const semester = (state = initialState, action: SemesterAction): SemesterState => {
+  switch (action.type) {
+    case SET_SEMESTERS:
+      return { ...state, semesters: action.semesters };
+    default:
+      return state;
+  }
+};
+
+export default semester;
diff --git a/src/reducers/common/track.js b/src/reducers/common/track.js
deleted file mode 100644
index a3ecf84..0000000
--- a/src/reducers/common/track.js
+++ /dev/null
@@ -1,18 +0,0 @@
-import { SET_TRACKS } from '../../actions/common/track';
-
-const initialState = {
-  tracks: null,
-};
-
-export const reducer = (state = initialState, action) => {
-  switch (action.type) {
-    case SET_TRACKS:
-      return Object.assign({}, state, {
-        tracks: action.tracks,
-      });
-    default:
-      return state;
-  }
-};
-
-export default reducer;
diff --git a/src/reducers/common/track.ts b/src/reducers/common/track.ts
new file mode 100644
index 0000000..6b4b350
--- /dev/null
+++ b/src/reducers/common/track.ts
@@ -0,0 +1,20 @@
+import { SET_TRACKS, TrackAction, Tracks } from '../../actions/common/track';
+
+interface TrackState {
+  tracks: Tracks | null;
+}
+
+const initialState: TrackState = {
+  tracks: null,
+};
+
+const track = (state = initialState, action: TrackAction): TrackState => {
+  switch (action.type) {
+    case SET_TRACKS:
+      return { ...state, tracks: action.tracks };
+    default:
+      return state;
+  }
+};
+
+export default track;
diff --git a/src/reducers/common/user.js b/src/reducers/common/user.ts
similarity index 53%
rename from src/reducers/common/user.js
rename to src/reducers/common/user.ts
index a65f34c..b36bc94 100644
--- a/src/reducers/common/user.js
+++ b/src/reducers/common/user.ts
@@ -1,16 +1,22 @@
-import { SET_USER, UPDATE_USER_REVIEW } from '../../actions/common/user';
+import User from '@/shapes/model/session/User';
+import { SET_USER, UPDATE_USER_REVIEW, UserAction } from '../../actions/common/user';
 
-const initialState = {
-  user: undefined,
+interface UserState {
+  user: User | null;
+}
+
+const initialState: UserState = {
+  user: null,
 };
 
-export const reducer = (state = initialState, action) => {
+const user = (state = initialState, action: UserAction): UserState => {
   switch (action.type) {
     case SET_USER:
-      return Object.assign({}, state, {
-        user: action.user,
-      });
+      return { ...state, user: action.user };
     case UPDATE_USER_REVIEW: {
+      if (!state.user) {
+        return state;
+      }
       const originalReviews = state.user.reviews;
       const { review } = action;
       const foundIndex = originalReviews.findIndex((r) => r.id === review.id);
@@ -22,16 +28,11 @@ export const reducer = (state = initialState, action) => {
               ...originalReviews.slice(foundIndex + 1, originalReviews.length),
             ]
           : [...originalReviews.slice(), review];
-      return Object.assign({}, state, {
-        user: {
-          ...state.user,
-          reviews: newReviews,
-        },
-      });
+      return { ...state, user: { ...state.user, reviews: newReviews } };
     }
     default:
       return state;
   }
 };
 
-export default reducer;
+export default user;