docx-js/src/file/paragraph/formatting/border.ts

// http://officeopenxml.com/WPborders.php
import { Attributes, XmlComponent } from "file/xml-components";

class Border extends XmlComponent {
    constructor() {
        super("w:bottom");
        this.root.push(
            new Attributes({
                color: "auto",
                space: "1",
                val: "single",
                sz: "6",
            }),
        );
    }
}

export class ThematicBreak extends XmlComponent {
    constructor() {
        super("w:pBdr");
        this.root.push(new Border());
    }
}