toJSON() {
    return Object.keys(this._combinationMatchers).reduce((memo, prefix) => {
      const combinationMatcher = this._combinationMatchers[prefix];

      memo[prefix] = combinationMatcher.toJSON();

      return memo;
    }, {});
  }
A plain JavaScript representation of the KeyMapMatcher, useful for
serialization or debugging
@returns {Object} Serialized representation of the key map matcher