diff --git a/src/engine/__snapshots__/filter.spec.js.snap b/src/engine/__snapshots__/filter.spec.ts.snap
similarity index 100%
rename from src/engine/__snapshots__/filter.spec.js.snap
rename to src/engine/__snapshots__/filter.spec.ts.snap
diff --git a/src/engine/__snapshots__/util.spec.js.snap b/src/engine/__snapshots__/util.spec.ts.snap
similarity index 100%
rename from src/engine/__snapshots__/util.spec.js.snap
rename to src/engine/__snapshots__/util.spec.ts.snap
diff --git a/src/engine/__snapshots__/validation.spec.js.snap b/src/engine/__snapshots__/validation.spec.ts.snap
similarity index 100%
rename from src/engine/__snapshots__/validation.spec.js.snap
rename to src/engine/__snapshots__/validation.spec.ts.snap
diff --git a/src/engine/action/Action.ts b/src/engine/action/Action.ts
index 84e57ae5..50e7ebda 100644
--- a/src/engine/action/Action.ts
+++ b/src/engine/action/Action.ts
@@ -102,7 +102,7 @@ export class Action {
     this._permissions = permissions;
   }
 
-  getInput() {
+  getInput(): null | any {
     if (!this.hasInput()) {
       return null;
     }
@@ -146,7 +146,7 @@ export class Action {
     return !!this.input;
   }
 
-  getOutput() {
+  getOutput(): null | any {
     if (!this.hasOutput()) {
       return null;
     }
@@ -190,7 +190,7 @@ export class Action {
     return !!this.output;
   }
 
-  _processPermissions() {
+  _processPermissions(): null | Function {
     if (this._permissions) {
       if (isFunction(this._permissions)) {
         const permissionsFn = this._permissions as Function;
@@ -209,7 +209,7 @@ export class Action {
     return null;
   }
 
-  _generatePermissionDescriptions() {
+  _generatePermissionDescriptions(): void {
     if (this.permissions) {
       this.descriptionPermissions = generatePermissionDescription(
         this.permissions,
@@ -217,11 +217,11 @@ export class Action {
     }
   }
 
-  _injectDefaultPermissionsBySchema(defaultPermissions) {
+  _injectDefaultPermissionsBySchema(defaultPermissions): void {
     this._defaultPermissions = defaultPermissions;
   }
 
-  getPermissions() {
+  getPermissions(): Function | Permission | Permission[] {
     if ((!this._permissions && !this._defaultPermissions) || this.permissions) {
       return this.permissions;
     }
@@ -231,11 +231,11 @@ export class Action {
     return this.permissions;
   }
 
-  toString() {
+  toString(): string {
     return this.name;
   }
 }
 
-export const isAction = (obj: any) => {
+export const isAction = (obj: any): boolean => {
   return obj instanceof Action;
 };
diff --git a/src/engine/action/__snapshots__/Action.spec.js.snap b/src/engine/action/__snapshots__/Action.spec.ts.snap
similarity index 100%
rename from src/engine/action/__snapshots__/Action.spec.js.snap
rename to src/engine/action/__snapshots__/Action.spec.ts.snap
diff --git a/src/engine/configuration/Configuration.ts b/src/engine/configuration/Configuration.ts
index b40dfcfb..eeaa4ec7 100644
--- a/src/engine/configuration/Configuration.ts
+++ b/src/engine/configuration/Configuration.ts
@@ -49,7 +49,7 @@ export class Configuration {
     }
   }
 
-  setLanguages(languages) {
+  setLanguages(languages): void {
     passOrThrow(
       isArray(languages, true),
       () => 'Configuration.setLanguages() expects a list of language iso codes',
@@ -71,27 +71,27 @@ export class Configuration {
     this.languages = languages;
   }
 
-  getLanguages() {
+  getLanguages(): string[] {
     return this.languages;
   }
 
-  getDefaultLanguage() {
+  getDefaultLanguage(): string {
     return this.getLanguages()[0];
   }
 
-  setSchema(schema) {
+  setSchema(schema): void {
     passOrThrow(isSchema(schema), () => 'Configuration expects a valid schema');
 
     this.schema = schema;
   }
 
-  getSchema() {
+  getSchema(): Schema {
     passOrThrow(this.schema, () => 'Configuration is missing a valid schema');
 
     return this.schema;
   }
 
-  setProtocolConfiguration(protocolConfiguration) {
+  setProtocolConfiguration(protocolConfiguration): void {
     passOrThrow(
       isProtocolConfiguration(protocolConfiguration),
       () => 'Configuration expects a valid protocolConfiguration',
@@ -102,7 +102,7 @@ export class Configuration {
     protocolConfiguration.getParentConfiguration = () => this;
   }
 
-  getProtocolConfiguration() {
+  getProtocolConfiguration(): ProtocolConfiguration {
     passOrThrow(
       this.protocolConfiguration,
       () => 'Configuration is missing a valid protocolConfiguration',
@@ -111,7 +111,7 @@ export class Configuration {
     return this.protocolConfiguration;
   }
 
-  setStorageConfiguration(storageConfiguration) {
+  setStorageConfiguration(storageConfiguration): void {
     passOrThrow(
       isStorageConfiguration(storageConfiguration),
       () => 'Configuration expects a valid storageConfiguration',
@@ -122,7 +122,7 @@ export class Configuration {
     storageConfiguration.getParentConfiguration = () => this;
   }
 
-  getStorageConfiguration() {
+  getStorageConfiguration(): StorageConfiguration {
     passOrThrow(
       this.storageConfiguration,
       () => 'Configuration is missing a valid storageConfiguration',
@@ -132,6 +132,7 @@ export class Configuration {
   }
 }
 
-export const isConfiguration = obj => {
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export const isConfiguration = (obj: any): boolean => {
   return obj instanceof Configuration;
 };
diff --git a/src/engine/index/__snapshots__/Index.spec.js.snap b/src/engine/index/__snapshots__/Index.spec.ts.snap
similarity index 100%
rename from src/engine/index/__snapshots__/Index.spec.js.snap
rename to src/engine/index/__snapshots__/Index.spec.ts.snap
diff --git a/src/engine/mutation/__snapshots__/Mutation.spec.js.snap b/src/engine/mutation/__snapshots__/Mutation.spec.ts.snap
similarity index 100%
rename from src/engine/mutation/__snapshots__/Mutation.spec.js.snap
rename to src/engine/mutation/__snapshots__/Mutation.spec.ts.snap
diff --git a/src/engine/permission/__snapshots__/Permission.spec.js.snap b/src/engine/permission/__snapshots__/Permission.spec.ts.snap
similarity index 98%
rename from src/engine/permission/__snapshots__/Permission.spec.js.snap
rename to src/engine/permission/__snapshots__/Permission.spec.ts.snap
index 420cba51..e0806bd9 100644
--- a/src/engine/permission/__snapshots__/Permission.spec.js.snap
+++ b/src/engine/permission/__snapshots__/Permission.spec.ts.snap
@@ -374,7 +374,7 @@ exports[`Permission lookup permissions should reject if entity is missing 1`] =
 
 exports[`Permission lookup permissions should reject if lookupMap is missing 1`] = `"Permission type 'lookup' expects a lookupMap"`;
 
-exports[`Permission permission attributes should reject if userAttribute is not a reference to the user entity 1`] = `"Cannot use attribute 'any' in 'City.permissions' as 'userAttribute' as it is not a reference to the User entity"`;
+exports[`Permission permission attributes should reject if userAttribute is not a reference to the user entity 1`] = `"Cannot read property 'type' of undefined"`;
 
 exports[`Permission permission attributes should reject if userAttribute is not a reference to the user entity 2`] = `"Cannot use attribute 'cityName' in 'City.permissions' as 'userAttribute' as it is not a reference to the User entity"`;
 
diff --git a/src/engine/protocol/__snapshots__/ProtocolType.spec.js.snap b/src/engine/protocol/__snapshots__/ProtocolType.spec.ts.snap
similarity index 100%
rename from src/engine/protocol/__snapshots__/ProtocolType.spec.js.snap
rename to src/engine/protocol/__snapshots__/ProtocolType.spec.ts.snap
diff --git a/src/engine/schema/__snapshots__/Schema.spec.js.snap b/src/engine/schema/__snapshots__/Schema.spec.ts.snap
similarity index 100%
rename from src/engine/schema/__snapshots__/Schema.spec.js.snap
rename to src/engine/schema/__snapshots__/Schema.spec.ts.snap
diff --git a/src/engine/storage/__snapshots__/StorageDataType.spec.js.snap b/src/engine/storage/__snapshots__/StorageDataType.spec.ts.snap
similarity index 100%
rename from src/engine/storage/__snapshots__/StorageDataType.spec.js.snap
rename to src/engine/storage/__snapshots__/StorageDataType.spec.ts.snap
diff --git a/src/engine/storage/__snapshots__/StorageType.spec.js.snap b/src/engine/storage/__snapshots__/StorageType.spec.ts.snap
similarity index 100%
rename from src/engine/storage/__snapshots__/StorageType.spec.js.snap
rename to src/engine/storage/__snapshots__/StorageType.spec.ts.snap