Hello, Filesystem or Web Local Storage! with emscripten_run_script_string version.

emscripten_run_script_with_web_storage.cxx

#include <iostream>
#include <fstream>
#include <string>

#ifdef EMSCRIPTEN
  #include <emscripten/emscripten.h>
#endif

namespace
{
  auto save( const std::string& path, const std::string& data )
    -> void
  {
#ifndef EMSCRIPTEN
    std::ofstream f;
    f.exceptions( std::ofstream::failbit | std::ofstream::badbit );
    f.open( path, std::ofstream::binary );
    f.write( data.data(), data.size() );
#else
    //EM_ASM_ARGS
    //( { localStorage.setItem( Pointer_stringify( $0 ), Pointer_stringify( $1 ) ); }
    //, path.data()
    //, data.data()
    //);
    std::string asm_code;
    asm_code += "localStorage.setItem( '";
    asm_code += path;
    asm_code += "', '";
    asm_code += data;
    asm_code += "' );";
    emscripten_run_script( asm_code.data() );
#endif
  }

  auto load( const std::string& path )
    -> std::string
  {
#ifndef EMSCRIPTEN
    std::ifstream f;
    f.exceptions( std::ifstream::failbit | std::ifstream::badbit );
    f.open( path, std::ifstream::binary );
    f.seekg( 0, std::ifstream::end );
    const auto size = f.tellg();
    f.seekg( 0, std::ifstream::beg );
    std::string buffer;
    buffer.resize( size );
    f.read( const_cast< char* >( buffer.data() ), size );
#else
    std::string asm_code;
    asm_code += "localStorage.getItem( '";
    asm_code += path;
    asm_code += "' );";
    std::string buffer = emscripten_run_script_string( asm_code.data() );
#endif
    return buffer;
  }
}

auto main()
  -> int
{
  const auto path = "example.data";
  const auto data = "Hello, Filesystem or Web Local Storage!";
  save( path, data );
  std::cout << load( path );
}

result.txt

Hello, Filesystem or Web Local Storage!