--- a/testing/marionette/sync.js
+++ b/testing/marionette/sync.js
@@ -61,21 +61,21 @@ const {TYPE_ONE_SHOT, TYPE_REPEATING_SLA
  *       } else {
  *         reject([]);
  *       }
  *     });
  * </pre></code>
  *
  * @param {Condition} func
  *     Function to run off the main thread.
- * @param {number=} timeout
+ * @param {number=} [timeout=2000] timeout
  *     Desired timeout.  If 0 or less than the runtime evaluation
  *     time of <var>func</var>, <var>func</var> is guaranteed to run
  *     at least once.  The default is 2000 milliseconds.
- * @param {number=} interval
+ * @param {number=} [interval=10] interval
  *     Duration between each poll of <var>func</var> in milliseconds.
  *     Defaults to 10 milliseconds.
  *
  * @return {Promise.<*>}
  *     Yields the value passed to <var>func</var>'s
  *     <code>resolve</code> or <code>reject</code> callbacks.
  *
  * @throws {*}