refactor: reuse signedInfoNode, use assertIsElementNode instead of cast + null check

Author: shunkica

src/signed-xml.ts

@@ -1099,13 +1099,8 @@ export class SignedXml {
     const signatureNamespace = "http://www.w3.org/2000/09/xmldsig#";
 
     // Find the SignedInfo element to append to
-    const signedInfoNode = xpath.select1(
-      `./*[local-name(.)='SignedInfo']`,
-      signatureElem,
-    ) as Element;
-    if (!signedInfoNode) {
-      throw new Error("Could not find SignedInfo element in signature");
-    }
+    const signedInfoNode = xpath.select1(`./*[local-name(.)='SignedInfo']`, signatureElem);
+    isDomNode.assertIsElementNode(signedInfoNode); // Type-safe assertion
 
     // Signature document is technically the same document as the one we are signing,
     // but we will extract it here for clarity (and also make it support detached signatures in the future)
@@ -1128,12 +1123,12 @@ export class SignedXml {
       // Process the reference
       for (const node of nodes) {
         isDomNode.assertIsElementNode(node);
+
         // Must not be a reference to Signature, SignedInfo, or a child of SignedInfo
-        const signedInfo = utils.findChildren(signatureElem, "SignedInfo")[0];
         if (
           node === signatureElem ||
-          node === signedInfo ||
-          utils.isDescendantOf(node, signedInfo)
+          node === signedInfoNode ||
+          utils.isDescendantOf(node, signedInfoNode)
         ) {
           throw new Error(
             `Cannot sign a reference to the Signature or SignedInfo element itself: ${ref.xpath}`,