feature: add support to add mutiple sections to the document inc. headers and footers

- write missing tests

src/file/document/body/body.ts

@@ -1,14 +1,59 @@
-import { XmlComponent } from "file/xml-components";
+import { XmlComponent, IXmlableObject } from "file/xml-components";
 import { SectionProperties, SectionPropertiesOptions } from "./section-properties/section-properties";
+import { Paragraph, ParagraphProperties } from "../..";
 
 export class Body extends XmlComponent {
+    private defaultSection: SectionProperties;
+
+    private sections: SectionProperties[] = [];
+
     constructor(sectionPropertiesOptions?: SectionPropertiesOptions) {
         super("w:body");
 
-        this.root.push(new SectionProperties(sectionPropertiesOptions));
+        this.defaultSection = new SectionProperties(sectionPropertiesOptions);
+        this.sections.push(this.defaultSection);
+    }
+
+    /**
+     * Adds new section properties.
+     * Note: Previous section is created in paragraph after the current element, and then new section will be added.
+     * The spec says:
+     *  - section element should be in the last paragraph of the section
+     *  - last section should be direct child of body
+     * @param section new section
+     */
+    addSection(section: SectionPropertiesOptions | SectionProperties) {
+        const currentSection = this.sections.pop() as SectionProperties;
+        this.root.push(this.createSectionParagraph(currentSection));
+        if (section instanceof SectionProperties) {
+            this.sections.push(section);
+        } else {
+            this.sections.push(new SectionProperties(section));
+        }
+    }
+    public prepForXml(): IXmlableObject {
+        if (this.sections.length === 1) {
+            this.root.push(this.sections[0]);
+        } else if (this.sections.length > 1) {
+            throw new Error("Invalid usage of sections. At the end of the body element there must be ONE section.");
+        }
+
+        return super.prepForXml();
     }
 
     public push(component: XmlComponent): void {
         this.root.push(component);
     }
+
+    get DefaultSection() {
+        return this.defaultSection;
+    }
+
+    private createSectionParagraph(section: SectionProperties) {
+        const paragraph = new Paragraph();
+        const properties = new ParagraphProperties();
+        properties.addChildElement(section);
+        paragraph.addChildElement(properties);
+        return paragraph;
+    }
 }