Introduce some functional programming techniques

src/file/content-types/content-types-attributes.ts

@@ -1,11 +1,11 @@
 import { XmlAttributeComponent } from "file/xml-components";
 
 export interface IContentTypeAttributes {
-    xmlns?: string;
+    readonly xmlns?: string;
 }
 
 export class ContentTypeAttributes extends XmlAttributeComponent<IContentTypeAttributes> {
-    protected xmlKeys = {
+    protected readonly xmlKeys = {
         xmlns: "xmlns",
     };
 }

src/file/content-types/default/default-attributes.ts

@@ -1,12 +1,12 @@
 import { XmlAttributeComponent } from "file/xml-components";
 
 export interface IDefaultAttributes {
-    contentType: string;
-    extension?: string;
+    readonly contentType: string;
+    readonly extension?: string;
 }
 
 export class DefaultAttributes extends XmlAttributeComponent<IDefaultAttributes> {
-    protected xmlKeys = {
+    protected readonly xmlKeys = {
         contentType: "ContentType",
         extension: "Extension",
     };

src/file/content-types/override/override-attributes.ts

@@ -1,12 +1,12 @@
 import { XmlAttributeComponent } from "file/xml-components";
 
 export interface IOverrideAttributes {
-    contentType: string;
-    partName?: string;
+    readonly contentType: string;
+    readonly partName?: string;
 }
 
 export class OverrideAttributes extends XmlAttributeComponent<IOverrideAttributes> {
-    protected xmlKeys = {
+    protected readonly xmlKeys = {
         contentType: "ContentType",
         partName: "PartName",
     };