unexpected token @ row: 2, column: 78

_createReducer.js

// @flow
function _createReducer<T, K>(handler: { [$PropertyType<K, 'type'>]: ({ action, state }: { action: K, state: T }) => T }): (state: T, action: K) => T {
    if (handler.hasOwnProperty(action.type))
        return handler[action.type]({
            action,
            state
        });
    return state;
}