As JavaScript is highly dynamic by nature, static information flow analyses are often too coarse to deal with the dynamic constructs of the language. To cope with this challenge, we present and prove the soundness of a new hybrid typing analysis for securing information flow in a JavaScript-like language. Our analysis combines static and dynamic typing in order to avoid rejecting programs due to imprecise typing information. Program regions that cannot be precisely typed at static time are wrapped inside an internal boundary statement used by the semantics to interleave the execution of statically verified code with the execution of code that must be dynamically checked.