how to build a js with series cpp file.if i build one by one,how to link them together

how to build a js with series cpp file.
if i build them one by one,how to link them together?

example

1.cpp

void webMain()
{
    extern void extern_func();
   extern_func();
}

2.cpp
void extern_func()
{
   client::console.log("extern_func!");
}

I need a method to compile file one by one.thus,when i change one of them,i need not to compile the whole project.
the example is compile .cpp -> .js
is there any way to compile .cpp(s)->.o(s)->.js
if it is already supported, which command should i use when generating a .o file,and which command for link?