Unit tests for new editor

src/editor/diff.js

@@ -57,7 +57,9 @@ export function diffDeletion(oldStr, newStr) {
  * @param {String} oldValue the previous value
  * @param {String} newValue the new value
  * @param {Number} caretPosition the position of the caret after `newValue` was applied.
- * @return {object}
+ * @return {object} an object with `at` as the offset where characters were removed and/or added,
+ *                  `added` with the added string (if any), and
+ *                  `removed` with the removed string (if any)
  */
 export function diffAtCaret(oldValue, newValue, caretPosition) {
     const diffLen = newValue.length - oldValue.length;