fix compatibility in `usePureJavaScript` mode

[boyxuper]

fix compatibility in usePureJavaScript mode, which globalScope can be undefined

[mattcollier]

Hello @boyxuper, thank you for the PR. Can you determine how it is that util.globalScope is returning undefined in your use case?

https://github.com/digitalbazaar/forge/blob/master/lib/util.js#L118

[boyxuper]

Yes, usePureJavaScript indicates a pure ES6 environment, which lack of window or self variables, and also not NodeJS runtime context. It's exactly my production environment, it's wechat miniapp runtime context.

[mattcollier]

Thanks for the additional detail. It sounds like to me that util.globalscope is returning the value of window which is undefined in your case.

Based on this one line: https://github.com/digitalbazaar/forge/blob/master/lib/util.js#L123

return typeof self === 'undefined' ? window : self;

I think adding the following conditional before L123 might work.

// applicable for wechat miniapp runtime context
if(!(window || self)) {
  return {};
}

Which I believe would eliminate the need for the additional code changes you've made.

[boyxuper]

This workaround is from this function _detectSubtleMsCrypto: https://github.com/digitalbazaar/forge/blob/master/lib/rsa.js#L1815

And, maybe the detection for undefined variables, should be kept this way? typeof self === 'undefined'

I worried that change type of a variable maybe too much for this.