Let the Stanza class extend the Builder class.

This enables better compatibility between the two and lets us use
Builder methods on Stanza classes.

src/builder.js

@@ -73,6 +73,15 @@ class Builder {
      * @property {string} [StanzaAttrs.xmlns]
      */
 
+    /** @type {Element} */
+    #nodeTree;
+    /** @type {Element} */
+    #node;
+    /** @type {string} */
+    #name;
+    /** @type {StanzaAttrs} */
+    #attrs;
+
     /**
      * The attributes should be passed in object notation.
      * @param {string} name - The name of the root element.
@@ -89,10 +98,35 @@ class Builder {
                 attrs = { xmlns: NS.CLIENT };
             }
         }
-        // Holds the tree being built.
-        this.nodeTree = xmlElement(name, attrs);
-        // Points to the current operation node.
-        this.node = this.nodeTree;
+
+        this.#name = name;
+        this.#attrs = attrs;
+    }
+
+    buildTree() {
+        return xmlElement(this.#name, this.#attrs);
+    }
+
+    /** @return {Element} */
+    get nodeTree() {
+        if (!this.#nodeTree) {
+            // Holds the tree being built.
+            this.#nodeTree = this.buildTree();
+        }
+        return this.#nodeTree;
+    }
+
+    /** @return {Element} */
+    get node() {
+        if (!this.#node) {
+            this.#node = this.tree();
+        }
+        return this.#node;
+    }
+
+    /** @param {Element} el */
+    set node(el) {
+        this.#node = el;
     }
 
     /**

src/stanza.js

@@ -1,3 +1,4 @@
+import Builder from './builder.js';
 import log from './log.js';
 import { getFirstElementChild, getParserError, xmlHtmlNode } from './utils.js';
 
@@ -32,36 +33,41 @@ export function toStanzaElement(string, throwErrorIfInvalidNS) {
 /**
  * A Stanza represents a XML element used in XMPP (commonly referred to as stanzas).
  */
-export class Stanza {
+export class Stanza extends Builder {
+
+    /** @type {string} */
+    #string;
+    /** @type {Array<string>} */
+    #strings;
+    /** @type {Array<string|Stanza>} */
+    #values;
+
     /**
      * @param {string[]} strings
      * @param {any[]} values
      */
     constructor(strings, values) {
-        this.strings = strings;
-        this.values = values;
+        super('stanza');
+        this.#strings = strings;
+        this.#values = values;
+    }
+
+    buildTree() {
+        return toStanzaElement(this.toString(), true);
     }
 
     /**
      * @return {string}
      */
     toString() {
-        this.string =
-            this.string ||
-            this.strings.reduce((acc, str) => {
-                const idx = this.strings.indexOf(str);
-                const value = this.values.length > idx ? this.values[idx] : '';
+        this.#string =
+            this.#string ||
+            this.#strings.reduce((acc, str) => {
+                const idx = this.#strings.indexOf(str);
+                const value = this.#values.length > idx ? this.#values[idx] : '';
                 return acc + str + (Array.isArray(value) ? value.join('') : value.toString());
             }, '');
-        return this.string;
-    }
-
-    /**
-     * @return {Element}
-     */
-    tree() {
-        this.node = this.node ?? toStanzaElement(this.toString(), true);
-        return this.node;
+        return this.#string;
     }
 }