optional location & options arguments

[rvagg]

ref #206 as per @mmalecki's suggestion

This gives two new constructor forms: levelup(options[, callback ]) and levelup(db[, callback ]).

If you don't provide a location argument but do provide an 'object' as your first argument then it must have a 'db' property that's a 'function' which is assumed to be a backend factory; otherwise it fails. The second form is where you don't provide a location argument but you do provide a 'function' as your first argument, this is assumed to be a back-end factory. In both cases a second argument is allowable, a callback 'function' but this is optional. We can't do a form where you provide a location as the first argument and a back-end factory as the second because we can't tell if it's supposed to be a factory or a callback, unless you provide a callback as well, but then, you're getting verbose anyway so it's just silly.

var levelup = require('levelup')
var memdown = require('memdown')

var db = levelup({ db: memdown })

// or

var db = levelup(memdown) 

required new DeferredLevelDOWN and MemDOWN releases to make the location optional.

[juliangruber]

+1

[mmalecki]

Why a factory and not just an instance?

[juliangruber]

@mmalecki because you can close and open a database. It even says in the readme that reopening a database is not adviced, so I'm inclined to drop that feature and pass in an instance here, way cleaner.

[ralphtheninja]

+1 for instance

[dominictarr]

so, when this is a leveldown instance, should it be preopened?
or will levelup call db.open?

[rvagg]

if it's not a factory then it'd need separate options for setting up the instance and setting up levelup

var db = levelup({ valueEncoding: 'json' }, leveldown({ createIfMissing: false }))

too messy IMO, I'm keen to stick with the factory.

Letting LevelUP manage instantiation gives it a chance to properly manage state for deferred-open operations, it knows exactly what state it's up to, it can use the callback to leveldown to trigger a completion to the open.

+1 to keeping factory, -1 to passing in an instance

[dominictarr]

an instance could be okay, if you have already opened the instance.
but otherwise, I think @rvagg has a fair point. We don't want to add more edge cases.

[mmalecki]

After reconsidering the open case, I'm +1 on factory. PR LGTM.

[juliangruber]

+1 for merging in, then MemDB can just be require('level-packager')(require('memdown')) and not have to provide a dummy database location.