Add JSDoc documentation for generic helper function callCallback.

src/utils/callcallback.js

@@ -3,6 +3,27 @@
 // This software is released under the MIT License.
 // https://opensource.org/licenses/MIT
 
+
+/**
+ * Most ml5 methods accept a callback function which will be
+ * called with the arguments (error, result).
+ *
+ * Generic type T describes the type of the result.
+ * @template T
+ * @callback ML5Callback<T>
+ * @param {unknown} error - any error thrown during the execution of the function.
+ * @param {T} [result] - the expected result, if successful.
+ * @return {void} - callbacks can have side effects, but should not return a value.
+ */
+
+/**
+ * Generic type T describes the type of the result, ie. the value that the Promise will resolve to.
+ * @template T
+ * @param {Promise<T>} promise - the Promise to resolve.
+ * @param {ML5Callback<T>} [callback] - optional callback function to be called
+ *     with the result or error from the resolved Promise.
+ * @return {Promise<T>} - returns the underlying Promise, which may be rejected.
+ */
 export default function callCallback(promise, callback) {
   if (!callback) return promise;
   return new Promise((resolve, reject) => {