Throw error if publicMethodFor is not found.

lib/DidKeyDriver.js

@@ -92,7 +92,11 @@ export class DidKeyDriver {
       throw new TypeError('The "purpose" parameter is required.');
     }
 
-    return didIo.findVerificationMethod({doc: didDocument, purpose});
+    const method = didIo.findVerificationMethod({doc: didDocument, purpose});
+    if(!method) {
+      throw new Error(`No verification method found for purpose "${purpose}"`);
+    }
+    return method;
   }
 
   /**

package.json

@@ -21,7 +21,7 @@
   "module": "lib/main.js",
   "dependencies": {
     "@digitalbazaar/did-io": "^1.0.0",
-    "@digitalbazaar/ed25519-verification-key-2020": "^2.0.0",
+    "@digitalbazaar/ed25519-verification-key-2020": "^2.1.1",
     "@digitalbazaar/x25519-key-agreement-key-2020": "^1.0.0",
     "did-context": "^3.0.0",
     "ed25519-signature-2020-context": "^1.0.1",

test/driver.spec.js

@@ -123,15 +123,23 @@ describe('did:key method driver', () => {
           'zB12NYF8RrR3h41TDCTJojY59usg3mbtbjnFs7Eud1Y6u');
     });
 
-    it('should return undefined if key is not found for purpose', async () => {
+    it('should throw error if key is not found for purpose', async () => {
       const did = 'did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH';
       // First, get the did document
       const didDocument = await didKeyDriver.get({did});
-      // Then publicMethodFor can be used to fetch key data
-      const result = didKeyDriver.publicMethodFor({
-        didDocument, purpose: 'invalidPurpose'
-      });
-      expect(result).to.be.undefined;
+
+      let error;
+      try {
+        didKeyDriver.publicMethodFor({
+          didDocument, purpose: 'invalidPurpose'
+        });
+      } catch(e) {
+        error = e;
+      }
+
+      expect(error).to.be.exist;
+      expect(error.message).to
+        .match(/No verification method found for purpose/);
     });
   });