Introduce some functional programming techniques

src/file/xml-components/attributes.ts

@@ -1,29 +1,29 @@
 import { XmlAttributeComponent } from "./default-attributes";
 
 export interface IAttributesProperties {
-    val?: string | number | boolean;
-    color?: string;
-    space?: string;
-    sz?: string;
-    type?: string;
-    rsidR?: string;
-    rsidRPr?: string;
-    rsidSect?: string;
-    w?: string;
-    h?: string;
-    top?: string;
-    right?: string;
-    bottom?: string;
-    left?: string;
-    header?: string;
-    footer?: string;
-    gutter?: string;
-    linePitch?: string;
-    pos?: string | number; // Little strange. Perhaps it is normal. Need to clarify in the spec.
+    readonly val?: string | number | boolean;
+    readonly color?: string;
+    readonly space?: string;
+    readonly sz?: string;
+    readonly type?: string;
+    readonly rsidR?: string;
+    readonly rsidRPr?: string;
+    readonly rsidSect?: string;
+    readonly w?: string;
+    readonly h?: string;
+    readonly top?: string;
+    readonly right?: string;
+    readonly bottom?: string;
+    readonly left?: string;
+    readonly header?: string;
+    readonly footer?: string;
+    readonly gutter?: string;
+    readonly linePitch?: string;
+    readonly pos?: string | number; // Little strange. Perhaps it is normal. Need to clarify in the spec.
 }
 
 export class Attributes extends XmlAttributeComponent<IAttributesProperties> {
-    protected xmlKeys = {
+    protected readonly xmlKeys = {
         val: "w:val",
         color: "w:color",
         space: "w:space",