diff --git a/backend/src/authentication/strategy.ts b/backend/src/authentication/strategy.ts
index 0993015..baea97f 100644
--- a/backend/src/authentication/strategy.ts
+++ b/backend/src/authentication/strategy.ts
@@ -100,7 +100,7 @@ class Strategy extends passport.Strategy {
 
   _loadProfile(accessToken: string) {
     const self = this;
-    var done = (err: Error | null, profile: any, info: any) => {
+    const done = (err: Error | null, profile: any, info: any) => {
       if (err) {
         self.error(err);
         return;
@@ -112,7 +112,7 @@ class Strategy extends passport.Strategy {
       self.success(profile, undefined);
     };
 
-    var verify = (err: Error, user: any) => {
+    const verify = (err: Error, user: any) => {
       if (err) {
         self.error(err);
         return;
@@ -124,7 +124,7 @@ class Strategy extends passport.Strategy {
   }
 
   _redirectToLogin() {
-    var uri = new URL(this.options.authorizationURL);
+    const uri = new URL(this.options.authorizationURL);
     uri.searchParams.append("response_type", "code");
     uri.searchParams.append("client_id", this.options.clientID);
     uri.searchParams.append("redirect_uri", this.options.callbackURL);
diff --git a/backend/src/services/rule.service.ts b/backend/src/services/rule.service.ts
index 701e2e7..15b9c12 100644
--- a/backend/src/services/rule.service.ts
+++ b/backend/src/services/rule.service.ts
@@ -72,7 +72,7 @@ export const toExplicitRules = (
   const explicitRules: ExplicitRule[] = [];
 
   for (const rule_i in rules) {
-    var current = new Date(from);
+    let current = new Date(from);
     const end = new Date(rules[rule_i].end_date);
     while (true) {
       if (dayApplies(current, rules[rule_i].day_mask)) {
@@ -120,7 +120,7 @@ const mergeIntoList = (
 };
 
 export const mergeRules = (rules: ExplicitRule[]): ExplicitRule[] => {
-  var mergedRules: ExplicitRule[] = [];
+  let mergedRules: ExplicitRule[] = [];
   for (const i in rules) {
     mergedRules = mergeIntoList(rules[i], mergedRules);
   }
@@ -137,7 +137,7 @@ const doesObeyRules = (rules: rule[], event: Event): Error | null => {
   const start = new Date(event.start);
   const end = new Date(event.end);
 
-  var explicitRules: ExplicitRule[] = mergeRules(
+  let explicitRules: ExplicitRule[] = mergeRules(
     toExplicitRules(rules, start, end),
   );
   for (const i in explicitRules) {
diff --git a/frontend/src/use-cases/rules/day-mask.element.jsx b/frontend/src/use-cases/rules/day-mask.element.jsx
index 8de59cf..f16f5fb 100644
--- a/frontend/src/use-cases/rules/day-mask.element.jsx
+++ b/frontend/src/use-cases/rules/day-mask.element.jsx
@@ -4,7 +4,7 @@ import "./day-mask.style.css";
 
 const dayArray = day_mask => {
   const days = [];
-  for (var i = 0; i < 7; i++) {
+  for (let i = 0; i < 7; i++) {
     days.push({ value: day_mask % 2, id: i });
     day_mask >>= 1;
   }