Add clear()

[vweevers]

Closes#648.

Blocked by Level/deferred-leveldown#72 and Level/encoding-down#89.

[vweevers]

Not 100% sure about the argument (options). I figured a listener might want to inspect the range options, to find out what was deleted. But that information is not exact (in contrast with the put, del and batch events that tell you exactly which keys were modified).

[vweevers]

@ralphtheninja Before we release this I want to think about adding a way to feature-detect it.

A module like noffle/kappa-view-level (used in cabal-core) may want to use db.clear() for its clearIndex(). If cabal-core were to switch to level-mem (instead of the old memdb that it currently uses) then the db it passes into kappa-view-level would indeed have a clear() method. Problem is, both these modules support passing in a custom db.

That db may be levelup with clear(), but its underlying db may not have clear(). So a check like typeof db.clear === 'function' would not suffice. Even worse, the db may be subleveldown with the default clear() of abstract-leveldown and delete keys outside of the sublevel range :(

[ralphtheninja]

Maybe we could start by adding a clear() to subleveldown to prevent from falling back to default behavior? But I guess the damage is already done there if it already falls back to using default clear().

[vweevers]

Yeah, we could make a patch release on subleveldown to either pin abstract-leveldown or add a noop _clear(). No one is using clear() yet and it isn't exposed on the outer levelup instance yet so we have time.

[vweevers]

It's time for manifests (again), I think. Level/community#42 (comment)

[ralphtheninja]

It's time for manifests (again), I think. Level/community#42 (comment)

I was thinking manifest just now :)

[vweevers]

I propose the following timeline:

1. Patch subleveldown
2. Release levelup minor with clear() (floats into level and friends)
3. Start working on manifests, as the "next big thing". In parallel, add clear() to subleveldown

Edit for the record: it turned out clear() worked out of the box: Level/subleveldown#73 (comment).