Flow: How do I tell flow that a script is in a web worker?

[rugulach]

I tried to typecheck a script that is normally run in a webworker context. importScripts works but postMessage fails. How do I tell flow that postMessage is valid in that context?

[philikon]

My recommendation would be to do:

declare var self: DedicatedWorkerGlobalScope;

And then use self.onmessage, self.importScripts, self.postMessage, etc. self is the global object in a web worker. The line above should tell Flow what type the variable this. DedicatedWorkerGlobalScope is a type already built into Flow and it defines importScripts, postMessage, etc.

That said, it'd be nice if we could tell Flow which environment a given piece of code is expected to run in, e.g. Node, browser, web worker, etc. so that it can define the right globals (and only those!). @gabelevi @samwgoldman @mroch what do guys think of optionally expanding the @flow pragma to allow environment specs, e.g. @flow env=node which would only enable the built-in core and node library definitions?

[rugulach]

@philikon thanks, the workaround passed the flow checks! But I really like your proposal for a project where some scripts run only in the browser and other scripts run only in node but some run in both places. for comparison, jshint has browser and node options controlling the globals.

[philikon]

Right, in my proposal, you'd be able to say @flow env=node,browser or @flow env=node,react or @flow env=browser,webworker.

[BlooJeans]

Just ran into an issue about the environment... in my case flow thought alert was defined (because of window.alert), but I hadn't defined it (in node).

Looking through the included type defs (https://github.com/facebook/flow/blob/7d700411a2ac83d4656e5d442e1c8a96f33341ff/lib/dom.js#L2982), you can see a whole bunch of 'unsafe' variable names that are included as part of a Browser, but don't necessarily exist in a node environment.

@philikon 's suggestion (or something similar in the .flowconfig) would be great to prevent my sort of mistake in the future

[benadamstyles]

Just had the same problem as @BlooJeans but with a function named focus, which is available in the browser but not in node.