Allow a union of libraries, for code that runs in multiple environments

Suggestion

🔍 Search Terms

webworker, dom, node, isomorphic

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Sometimes code is designed to run in multiple environments. For example, code may run in both nodejs and the browser, or a web page and a web worker.

Currently, you can use tsconfig to say your environment is both a 'DOM' and 'webworker', but that's never true in the wild. It's 'DOM' or 'webworker'.

Take this example (playground link):

localStorage.set('foo', 'bar');
importScripts('hello.js');

TypeScript is fine with this if you include both 'DOM' and 'webworker' libs, but this code will fail in a webworker, because localStorage doesn't exist, and it will fail in a page, because importScripts doesn't exist.

This would be solved by a feature that allows developers to specify the environment as A or B. The code above would show errors, because localStoage and importScripts may not exist.

📃 Motivating Example

Hopefully the above description provides this.

💻 Use Cases

Hopefully the above description provides this.

The issue does not name implementation files or tests. Start by reviewing the tsconfig library-selection behavior and the linked Playground example, then examine how the TypeScript compiler represents environment libraries. Done should include a defined A-or-B environment model and type checking that rejects APIs unavailable in either selected environment.