Introduce some functional programming techniques

src/file/footnotes/footnotes-attributes.ts

@@ -1,27 +1,27 @@
 import { XmlAttributeComponent } from "file/xml-components";
 
 export interface IFootnotesAttributesProperties {
-    wpc?: string;
-    mc?: string;
-    o?: string;
-    r?: string;
-    m?: string;
-    v?: string;
-    wp14?: string;
-    wp?: string;
-    w10?: string;
-    w?: string;
-    w14?: string;
-    w15?: string;
-    wpg?: string;
-    wpi?: string;
-    wne?: string;
-    wps?: string;
-    Ignorable?: string;
+    readonly wpc?: string;
+    readonly mc?: string;
+    readonly o?: string;
+    readonly r?: string;
+    readonly m?: string;
+    readonly v?: string;
+    readonly wp14?: string;
+    readonly wp?: string;
+    readonly w10?: string;
+    readonly w?: string;
+    readonly w14?: string;
+    readonly w15?: string;
+    readonly wpg?: string;
+    readonly wpi?: string;
+    readonly wne?: string;
+    readonly wps?: string;
+    readonly Ignorable?: string;
 }
 
 export class FootnotesAttributes extends XmlAttributeComponent<IFootnotesAttributesProperties> {
-    protected xmlKeys = {
+    protected readonly xmlKeys = {
         wpc: "xmlns:wpc",
         mc: "xmlns:mc",
         o: "xmlns:o",