Make library import safe in non-browser environments

[chetansastry]

window?.MutationObserver raises a ReferenceError when window is not defined (e.g. in Node.js). This code path gets hit just on import. This can be problematic particularly in unit tests and SSR.

Added a typeof guard which should make it not raise error on import.

[CLAassistant]

All committers have signed the CLA.

[ajhyndman]

@chetansastry Thank you for the contribution!

For what it's worth, I am pretty confident that this code path won't be hit until you call init(), rather than on import. That should mean you could also work around this by wrapping your init() call with a try/catch.

With that said, I don't see any reason not to add the extra guard!

[chetansastry]

Thanks for accepting!

Here's a testcase to demonstrate the problem (should have included as part of PR, but oh well).

1. Navigate to https://npm.runkit.com/%40dropbox%2Fttvc (version 2.2.1)
2. Hit run (just import, no init())

You'll see a ReferenceError: window is not defined since Runkit is node environment without a window object.

[ajhyndman]

Oh, thank you for sharing the test case!

I see it now. This is because the index file statically invokes getNetworkIdleObservable when exporting the increment/decrement methods.

That shouldn't be the case, so I'll push another fix to address that.