Write data to a file after Html5 launch

I have a project where I write some data in a file in Unreal and it works fine. Now I want to launch my project as Html5 and do the same stuff there.
My function in c++ uses the FFileHelper::SaveStringToFile Method, but this seems not to work after the Html launch . Now I am curious if there is a way to save data to a file after launching an Ue4 project as Html.

HTML5 File API is used to access files on the client machine via web browsers. There are 3 levels of File APIs in HTML5 standards:

File API (Let me call it as File Reader API) 
File API: Writer (also called as File Writer API) 
File API: Directories and System (also called as File System API)  

I spends some time on analyzing the capabilities of each level and the compatibility with different web browsers. This would be helpful to choose technologies for my future web application development which may access the client-side local files, for example, uploading and downloading files.

No browser will let you willy nilly write or read from the host computer. That would cause a great deal of security problems…
File APIs are not designed for the purpose you want to achieve here.

You can check HTML5 web storage (also known as localStorage object). You can store a limited amount of data.

This will require you learning javascript, editing current html page template of UE4, and probably messing around with EMCC (emscripten).
Also, you will definitely need to write some C++ code to wrap these javascript calls for your project…

Local Storage - from MDN
Emscripten - Interacting with javascript code