Changes to make easier to use PouchDB on Node on Windows

[jacargentina]

Implements needed changes for #3110

[nolanlawson]

This seems to be failing due to jshint, but I really like the approach. I'm +1 after jshint fixes and a green run. Also squashing the commits would be super. :)

@calvinmetcalf could you comment on this "optional dependency" stuff? Looks fine to me; users are at least warned about the lack of a leveldown dependency, although maybe we should emit('error') or something?

[NickColley]

This is a great addition thanks @jacargentina, I've had problems building in Windows in the past too, when you're just doing browserify stuff it's a pain.

[daleharvey]

Huge +1 for this, will wait for calvins response, seems like we dont need the try {} since nothing should throw with the require.resolve but otherwise will be very happy to get this in

[jacargentina]

@daleharvey i've looked at

http://stackoverflow.com/a/15303236/2917185

It seems it throws an error, when the module is not found

[marten-de-vries]

I like the idea, but isn't that require.resolve if statement useless? My understanding: the file adapters/leveldb.js will always be there. Leveldown might not be, but that error will only be thrown by the require call inside the if statement (or actually by the code that is required). Correct?

[jacargentina]

Pouch will not work at all if the IF is removed. Try it!

[marten-de-vries]

@jacargentina It works here. Maybe we're doing something different?

This might be even more flexible if only the require() of leveldown in leveldb.js would be wrapped in a try/catch, then e.g. memdown/sqldown/etc. would still work. I'll see if I can get a branch with that up...

[nolanlawson]

This might be even more flexible if only the require() of leveldown in leveldb.js would be wrapped in a try/catch, then e.g. memdown/sqldown/etc. would still work. I'll see if I can get a branch with that up...

+1, this is definitely needed. E.g. @yaronyg has been trying to build PouchDB on exotic embedded platforms, and needed a way to just use SQLdown or just use medeadown etc.

[marten-de-vries]

OK, so the branch that probably was just linked to by github produces the following:

marten@marten-laptop:~/git/pouchdb$ node
> var PouchDB = require('./')
undefined
> var db = new PouchDB('test')
undefined
> db.allDocs().catch(console.log.bind(console))
[object Object]
> [Error: leveldown not available (specify another backend using the db option)]

undefined
> var db2 = new PouchDB('test', {db: require('memdown')})
undefined
> db2.allDocs().then(console.log.bind(console))
[object Object]
> { total_rows: 0, offset: 0, rows: [] }

undefined
> 

Might be better if the var db = new PouchDB('test') line already threw an exception, but this seems in line with how websql handles an error at this stage of the instantiation process. Plus, I wasn't sure how to do that this quickly.

[calvinmetcalf]

wrap this in a (function () {}()) to avoid v8 deopt

[jacargentina]

Somebody please take the idea and code as discussed/suggested... I dont have that much knowledge, v8 deopt thing.. Etc feel free to close this PR and do your own. Hope this gets merged and next release!

[nolanlawson]

Calvin's referring to the fact that try/catch gets de-optimized unless you put it inside its own function. We got your back, though, no prob. :)

[marten-de-vries]

I just opened #3140 which does the try/catch wrapping & also supports other leveldb backends like memdown.

[marten-de-vries]

#3140 has been merged, so I'm closing this. Thanks @jacargentina for the initial implementation & everyone for the input on this.