diff --git a/src/main/angular/src/app/api/api.service.ts b/src/main/angular/src/app/api/api.service.ts
index 10046b8..2e6ee91 100644
--- a/src/main/angular/src/app/api/api.service.ts
+++ b/src/main/angular/src/app/api/api.service.ts
@@ -40,7 +40,7 @@ export class ApiService {
       .subscribe(next);
   }
 
-  getNone<T>(path: any[], next: Next<void> | undefined = undefined): Subscription {
+  getNone<T>(path: any[], next?: Next<void>): Subscription {
     return this.http.get<void>(getUrl('http', path)).subscribe(next);
   }
 
@@ -48,7 +48,7 @@ export class ApiService {
     return this.http.get(getUrl('http', path), {responseType: "text"}).subscribe(next);
   }
 
-  getSingle<T>(path: any[], fromJson: FromJson<T>, next: Next<T> | undefined = undefined): Subscription {
+  getSingle<T>(path: any[], fromJson: FromJson<T>, next?: Next<T>): Subscription {
     return this.http.get<any>(getUrl('http', path)).pipe(map(fromJson)).subscribe(next);
   }
 
@@ -56,11 +56,11 @@ export class ApiService {
     return this.http.get<any[]>(getUrl('http', path)).pipe(map(list => list.map(fromJson))).subscribe(next);
   }
 
-  postNone(path: any[], data: any, next: Next<void> | undefined = undefined): Subscription {
+  postNone(path: any[], data: any, next?: Next<void>): Subscription {
     return this.http.post<void>(getUrl('http', path), data).subscribe(next);
   }
 
-  postSingle<T>(path: any[], data: any, fromJson: FromJson<T>, next: Next<T> | undefined = undefined): Subscription {
+  postSingle<T>(path: any[], data: any, fromJson: FromJson<T>, next?: Next<T>): Subscription {
     return this.http.post<any>(getUrl('http', path), data).pipe(map(fromJson)).subscribe(next);
   }
 
diff --git a/src/main/angular/src/app/api/series/Value.ts b/src/main/angular/src/app/api/series/Value.ts
index 904c6a1..78118f6 100644
--- a/src/main/angular/src/app/api/series/Value.ts
+++ b/src/main/angular/src/app/api/series/Value.ts
@@ -30,7 +30,7 @@ export class Value implements IValue {
   }
 
   static map(iValue: IValue | null, mapping: GetValue): Value | null {
-    if (!iValue || !iValue.value) {
+    if (iValue === null || iValue.value === null) {
       return null;
     }
     const value = mapping(iValue.value);
@@ -54,14 +54,14 @@ export class Value implements IValue {
   }
 
   static bi(a: IValue | null, b: IValue | null, combineValue: CombineValue) {
-    if (!a) {
+    if (a === null) {
       return new Value(null, null, b?.unit || null);
     }
-    if (!b) {
+    if (b === null) {
       return new Value(null, null, a?.unit || null);
     }
-    const oldestDate = !a.date || !b.date ? null : dateMin(a.date, b.date);
-    const difference = !a.value || !b.value ? null : combineValue(a.value, b.value);
+    const oldestDate = a.date === null || b.date === null ? null : dateMin(a.date, b.date);
+    const difference = a.value === null || b.value === null ? null : combineValue(a.value, b.value);
     return new Value(oldestDate, difference, a.unit);
   }
 
diff --git a/src/main/angular/src/app/api/validators.ts b/src/main/angular/src/app/api/validators.ts
index c76bc5e..79d2238 100644
--- a/src/main/angular/src/app/api/validators.ts
+++ b/src/main/angular/src/app/api/validators.ts
@@ -1,42 +1,42 @@
 import {FromJson} from "./types";
 
 export function validateNumber(json: any): number {
-  if (!(typeof json == "number")) {
+  if (!(typeof json === "number")) {
     throw new Error("Not a number: " + json + " (" + typeof json + "): " + JSON.stringify(json));
   }
   return json;
 }
 
 export function validateNumberOrNull(json: any): number | null {
-  if (json == null) {
+  if (json === null || json === undefined) {
     return null;
   }
   return validateNumber(json);
 }
 
 export function validateBoolean(json: any): boolean {
-  if (!(typeof json == "boolean")) {
+  if (!(typeof json === "boolean")) {
     throw new Error("Not a boolean: " + json + " (" + typeof json + "): " + JSON.stringify(json));
   }
   return json;
 }
 
 export function validateBooleanOrNull(json: any): boolean | null {
-  if (json == null) {
+  if (json === null || json === undefined) {
     return null;
   }
   return validateBoolean(json);
 }
 
 export function validateString(json: any): string {
-  if (!(typeof json == "string")) {
+  if (!(typeof json === "string")) {
     throw new Error("Not a string: " + json + " (" + typeof json + "): " + JSON.stringify(json));
   }
   return json;
 }
 
 export function validateStringOrNull(json: any): string | null {
-  if (json == null) {
+  if (json === null || json === undefined) {
     return null;
   }
   return validateString(json);
@@ -47,7 +47,7 @@ export function validateDate(json: any): Date {
 }
 
 export function validateDateOrNull(json: any): Date | null {
-  if (json == null) {
+  if (json === null || json === undefined) {
     return null;
   }
   return validateDate(json);
@@ -58,7 +58,7 @@ export function validateList<T>(json: any, fromJson: FromJson<T>): T[] {
 }
 
 export function validateLocalDateOrNull(json: any): Date | null {
-  if (json == null) {
+  if (json === null || json === undefined) {
     return null;
   }
   return validateLocalDate(json);
diff --git a/src/main/angular/src/app/pages/dashboard/dashboard.component.ts b/src/main/angular/src/app/pages/dashboard/dashboard.component.ts
index 7e8af9a..8580c87 100644
--- a/src/main/angular/src/app/pages/dashboard/dashboard.component.ts
+++ b/src/main/angular/src/app/pages/dashboard/dashboard.component.ts
@@ -57,7 +57,7 @@ export class DashboardComponent implements OnInit {
     if (fresh.name !== name) {
       return old;
     }
-    if (old == null || old.lastDate.getTime() <= fresh.lastDate.getTime()) {
+    if (old === null || old.lastDate.getTime() <= fresh.lastDate.getTime()) {
       return fresh;
     }
     return old;