How to share code between TypeScript projects?

Since Typescript 3.0 this can be done with Project References.

Typescript docs: https://www.typescriptlang.org/docs/handbook/project-references.html

I believe you would have to move lib.ts into a small ts project called something like ‘lib’

The lib project should have a tsconfig containing:

// lib/tsconfig.json
    {
          "compilerOptions": {
            /* Truncated compiler options to list only relevant options */
            "declaration": true, 
            "declarationMap": true,
            "rootDir": ".",   
            "composite": true,     
          },
          "references": []  // * Any project that is referenced must itself have a `references` array (which may be empty).
        }

Then in both project-a and project-b add the reference to this lib project into your tsconfig

// project-a/ts-config.json
// project-b/ts-config.json
{
        "compilerOptions": {
            "target": "es5", 
            "module": "es2015",
            "moduleResolution": "node"
            // ...
        },
        "references": [
            { 
                "path": "../lib",
                // add 'prepend' if you want to include the referenced project in your output file
                "prepend": true, 
            }
        ]
    }

In the lib project. Create a file index.ts which should export all your code you want to share with other projects.

// lib/index.ts    
export * from 'lib.ts';

Now, let’s say lib/lib.ts looks like this:

// lib/lib.ts
export const log = (message: string) => console.log(message);

You can now import the log function from lib/lib.ts in both project-a and project-b

// project-a/app.ts 
// project-b/app.ts
import { log } from '../lib';
log("This is a message");

Before your intelissense will work, you now need to build both your project-a and project-b using:

tsc -b 

Which will first build your project references (lib in this case) and then build the current project (project-a or project-b).

The typescript compiler will not look at the actual typescript files from lib. Instead it will only use the typescript declaration files (*.d.ts) generated when building the lib project.

That’s why your lib/tsconfig.json file must contain:

"declaration": true,

However, if you navigate to the definition of the log function in project-a/app.ts using F12 key in Visual Studio code, you’ll be shown the correct typescript file.
At least, if you have correctly setup your lib/tsconfig.json with:

"declarationMap": true,

I’ve create a small github repo demonstrating this example of project references with typescript:

https://github.com/thdk/TS3-projects-references-example