feat(jsonld): Add method to find node in document

The DID public key information may be embedded in the DID document,
so add the "findNodeById()" function to find the node in the
document.

See also: https://w3c-ccg.github.io/did-spec/#public-keys

packages/jsonld/README.md

@@ -33,6 +33,8 @@ yarn add @tangleid/jsonld
 
     * [~frame(input, frame, [options])](#module_jsonld..frame)
 
+    * [~findNodeById(document, id)](#module_jsonld..findNodeById)
+
     * [~generateRsaKeyPair([options])](#module_jsonld..generateRsaKeyPair)
 
     * [~signRsaSignature(document, publicKey, privateKeyPem, [options])](#module_jsonld..signRsaSignature)
@@ -148,6 +150,18 @@ Performs JSON-LD flattening.
 Performs JSON-LD framing.
 
 **Returns**: <code>Promise.&lt;object&gt;</code> - Promise that resolves to the framed output.  
+<a name="module_jsonld..findNodeById"></a>
+
+### *jsonld*~findNodeById(document, id)
+
+| Param | Type | Description |
+| --- | --- | --- |
+| document | <code>object</code> | the JSON-LD document to find. |
+| id | <code>string</code> | String that specifies the ID value. |
+
+Returns the first node with the specified value of the ID attribute.
+
+**Returns**: <code>Promise.&lt;(object\|null)&gt;</code> - Promise that resolves to the node.  
 <a name="module_jsonld..generateRsaKeyPair"></a>
 
 ### *jsonld*~generateRsaKeyPair([options])

packages/jsonld/src/jsonld.ts

@@ -123,3 +123,29 @@ export const frame = async (input: object, frame: object, options: object = {}):
   // @ts-ignore
   return jsonld.frame(input, frame, options);
 };
+
+/**
+ * Returns the first node with the specified value of the ID attribute.
+ * @function findNodeById
+ * @param {object} document - the JSON-LD document to find.
+ * @param {string} id - String that specifies the ID value.
+ * @return {Promise<object|null>} Promise that resolves to the node.
+ */
+export const findNodeById = async (document: object, id: string): Promise<object | null> => {
+  const flattened: { [index: string]: any } = await flatten(document, {});
+
+  if (!flattened.hasOwnProperty('@graph')) {
+    throw new Error('@graph property does not exist in the flattened document');
+  }
+
+  const graph = flattened['@graph'] as Array<{ [index: string]: any }>;
+  const node = graph.find(element => {
+    return element['@id'] === id;
+  });
+
+  if (node === undefined) {
+    return null;
+  }
+
+  return node;
+};