Just referencing another project isn't enough for the TypeScript compiler to properly handle multiple projects. We need to add some additional compiler options in the dependent project.

The compilerOptions field has a new field called composite, which must be set to true if we are using multiple projects. This ensures certain options are enabled so that this project can be referenced and built incrementally for any project that depends on it.

When composite is true, declaration must also be set to true, forcing the corresponding .d.ts file to be generated, containing the project's types. This allows TypeScript to only build dependent projects when types are changed and not rebuild all the dependent projects all the time.

Let's make the following changes to our working example:

1. Let's open up tsconfig.json in the Shared project and make the following changes:

{
  "compilerOptions": {
    "composite": true,
    "declaration": true,
    ...
  },
}

2. In the terminal, let's go to the Shared project directory and compile our Shared project:

cd ..
cd Shared
tsc

The project compiles okay. Let's now try to compile ProjectA again in the terminal:

cd ..
cd ProjectA
tsc

   This time, ProjectA compiles just fine.

So, we have successfully tied together two projects using TypeScript 3's multiple projects feature. In the next section, we'll improve the setup of our projects even more.