6 files changed, 104 insertions, 10 deletions
diff --git a/src/definitions/definitions.ts b/src/definitions/definitions.ts
index 413b2bd7..87399408 100644
--- a/src/definitions/definitions.ts
+++ b/src/definitions/definitions.ts
@@ -52,4 +52,4 @@ export type LocaleDefinition = {
   system?: SystemDefinitions;
   vehicle?: VehicleDefinitions;
   word?: WordDefinitions;
-} & Record<string, Record<string, unknown>>;
+} & Record<string, Record<string, unknown> | undefined>;
diff --git a/src/faker.ts b/src/faker.ts
index a694fcc2..e715234d 100644
--- a/src/faker.ts
+++ b/src/faker.ts
@@ -3,6 +3,8 @@ import { FakerError } from './errors/faker-error';
 import { deprecated } from './internal/deprecated';
 import type { Mersenne } from './internal/mersenne/mersenne';
 import mersenne from './internal/mersenne/mersenne';
+import type { LocaleProxy } from './locale-proxy';
+import { createLocaleProxy } from './locale-proxy';
 import { AirlineModule } from './modules/airline';
 import { AnimalModule } from './modules/animal';
 import { ColorModule } from './modules/color';
@@ -59,7 +61,8 @@ import { mergeLocales } from './utils/merge-locales';
  * customFaker.music.genre(); // throws Error as this data is not available in `es`
  */
 export class Faker {
-  readonly definitions: LocaleDefinition;
+  readonly rawDefinitions: LocaleDefinition;
+  readonly definitions: LocaleProxy;
   private _defaultRefDate: () => Date = () => new Date();
 
   /**
@@ -329,7 +332,8 @@ export class Faker {
       locale = mergeLocales(locale);
     }
 
-    this.definitions = locale as LocaleDefinition;
+    this.rawDefinitions = locale as LocaleDefinition;
+    this.definitions = createLocaleProxy(this.rawDefinitions);
   }
 
   /**
diff --git a/src/locale-proxy.ts b/src/locale-proxy.ts
new file mode 100644
index 00000000..c5c6aa1b
--- /dev/null
+++ b/src/locale-proxy.ts
@@ -0,0 +1,91 @@
+import type { LocaleDefinition } from './definitions';
+import { FakerError } from './errors/faker-error';
+
+/**
+ * A proxy for LocaleDefinitions that marks all properties as required and throws an error when an entry is accessed that is not defined.
+ */
+export type LocaleProxy = Readonly<{
+  [key in keyof LocaleDefinition]-?: Readonly<
+    Required<NonNullable<LocaleDefinition[key]>>
+  >;
+}>;
+
+const throwReadOnlyError: () => never = () => {
+  throw new FakerError('You cannot edit the locale data on the faker instance');
+};
+
+/**
+ * Creates a proxy for LocaleDefinition that throws an error if an undefined property is accessed.
+ *
+ * @param locale The locale definition to create the proxy for.
+ */
+export function createLocaleProxy(locale: LocaleDefinition): LocaleProxy {
+  const proxies = {} as LocaleDefinition;
+  return new Proxy(locale, {
+    has(): true {
+      // Categories are always present (proxied), that's why we return true.
+      return true;
+    },
+
+    get(
+      target: LocaleDefinition,
+      categoryName: keyof LocaleDefinition
+    ): LocaleDefinition[keyof LocaleDefinition] {
+      if (categoryName in proxies) {
+        return proxies[categoryName];
+      }
+
+      return (proxies[categoryName] = createCategoryProxy(
+        categoryName,
+        target[categoryName]
+      ));
+    },
+
+    set: throwReadOnlyError,
+    deleteProperty: throwReadOnlyError,
+  }) as LocaleProxy;
+}
+
+/**
+ * Creates a proxy for a category that throws an error when accessing an undefined property.
+ *
+ * @param categoryName The name of the category.
+ * @param categoryData The module to create the proxy for.
+ */
+function createCategoryProxy<
+  CategoryData extends Record<string | symbol, unknown>
+>(
+  categoryName: string,
+  categoryData: CategoryData = {} as CategoryData
+): Required<CategoryData> {
+  return new Proxy(categoryData, {
+    has(target: CategoryData, entryName: keyof CategoryData): boolean {
+      const value = target[entryName];
+      return value != null;
+    },
+
+    get(
+      target: CategoryData,
+      entryName: keyof CategoryData
+    ): CategoryData[keyof CategoryData] {
+      const value = target[entryName];
+      if (value === null) {
+        throw new FakerError(
+          `The locale data for '${categoryName}.${entryName.toString()}' aren't applicable to this locale.
+  If you think this is a bug, please report it at: https://github.com/faker-js/faker`
+        );
+      } else if (value === undefined) {
+        throw new FakerError(
+          `The locale data for '${categoryName}.${entryName.toString()}' are missing in this locale.
+  Please contribute the missing data to the project or use a locale/Faker instance that has these data.
+  For more information see https://next.fakerjs.dev/guide/localization.html`
+        );
+      } else {
+        return value;
+      }
+    },
+
+    set: throwReadOnlyError,
+    deleteProperty: throwReadOnlyError,
+  }) as Required<CategoryData>;
+}
diff --git a/src/modules/helpers/index.ts b/src/modules/helpers/index.ts
index 48c8cea0..0bb54fb6 100644
--- a/src/modules/helpers/index.ts
+++ b/src/modules/helpers/index.ts
@@ -1160,7 +1160,7 @@ export class HelpersModule {
     const parts = method.split('.');
 
     let currentModuleOrMethod: unknown = this.faker;
-    let currentDefinitions: unknown = this.faker.definitions;
+    let currentDefinitions: unknown = this.faker.rawDefinitions;
 
     // Search for the requested method or definition
     for (const part of parts) {
diff --git a/src/modules/location/index.ts b/src/modules/location/index.ts
index c39711b7..26ead8ad 100644
--- a/src/modules/location/index.ts
+++ b/src/modules/location/index.ts
@@ -73,8 +73,7 @@ export class LocationModule {
     const { state } = options;
 
     if (state) {
-      const zipRange =
-        this.faker.definitions.location.postcode_by_state?.[state];
+      const zipRange = this.faker.definitions.location.postcode_by_state[state];
 
       if (zipRange) {
         return String(this.faker.number.int(zipRange));
diff --git a/src/modules/person/index.ts b/src/modules/person/index.ts
index 789cd58c..428b1373 100644
--- a/src/modules/person/index.ts
+++ b/src/modules/person/index.ts
@@ -102,7 +102,7 @@ export class PersonModule {
    */
   firstName(sex?: SexType): string {
     const { first_name, female_first_name, male_first_name } =
-      this.faker.definitions.person;
+      this.faker.rawDefinitions.person ?? {};
 
     return selectDefinition(this.faker, this.faker.helpers.arrayElement, sex, {
       generic: first_name,
@@ -132,7 +132,7 @@ export class PersonModule {
       last_name_pattern,
       male_last_name_pattern,
       female_last_name_pattern,
-    } = this.faker.definitions.person;
+    } = this.faker.rawDefinitions.person ?? {};
 
     if (
       last_name_pattern != null ||
@@ -174,7 +174,7 @@ export class PersonModule {
    */
   middleName(sex?: SexType): string {
     const { middle_name, female_middle_name, male_middle_name } =
-      this.faker.definitions.person;
+      this.faker.rawDefinitions.person ?? {};
 
     return selectDefinition(this.faker, this.faker.helpers.arrayElement, sex, {
       generic: middle_name,
@@ -315,7 +315,7 @@ export class PersonModule {
    */
   prefix(sex?: SexType): string {
     const { prefix, female_prefix, male_prefix } =
-      this.faker.definitions.person;
+      this.faker.rawDefinitions.person ?? {};
 
     return selectDefinition(this.faker, this.faker.helpers.arrayElement, sex, {
       generic: prefix,