Create a manifest describing the abilities of a levelup or abstract-leveldown db
Last updated 8 months ago by vweevers .
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Create a manifest describing the abilities of a levelup or abstract-leveldown db.

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const supports = require('level-supports')

db.supports = supports({
  bufferKeys: true,
  additionalMethods: {
    approximateSize: true

Receivers of the db can then use it like so:

if (!db.supports.permanence) {
  throw new Error('Persistent storage is required')

if (db.supports.bufferKeys && db.supports.promises) {
  await db.put(Buffer.from('key'), 'value')


manifest = supports([manifest, ..])

Given zero or more manifest objects, returns a merged and enriched manifest object that has:

  • Truthy properties for each of the features listed below
  • An additionalMethods object

For future extensibility, the properties are truthy rather than strictly typed booleans. Falsy or absent properties are converted to false, other values are allowed:

supports().streams // false
supports({ streams: true }).streams // true
supports({ streams: {} }).streams // {}
supports({ streams: 1 }, { streams: 2 }).streams // 2

For consumers of the manifest this means they should check support like so:

if (db.supports.streams)

Rather than:

if (db.supports.streams === true)

Note: the manifest describes high-level features that typically encompass multiple methods of a db. It is currently not a goal to describe a full API, or versions of it.


bufferKeys (boolean)

Does the db support Buffer keys? May depend on runtime environment as well. Does not include support of other binary types like typed arrays (which is why this is called bufferKeys rather than binaryKeys).

snapshots (boolean)

Does the db have snapshot guarantees (meaning that reads are unaffected by simultaneous writes)? Must be false if any of the following is true:

  • Reads don't operate on a snapshot
  • Snapshots are created asynchronously.

permanence (boolean)

Does data survive after process exit? Is false for e.g. memdown, typically true.

seek (boolean)

Does db.iterator() support seek(..)?

clear (boolean)

Does db support db.clear(..)? For an overview, see Level/community#79.

status (boolean)

Does db have a status property? Currently available on abstract-leveldown implementations, but not levelup.

deferredOpen (boolean)

Can operations like db.put() be called without explicitly opening the db? Like so:

var db = level()
db.put('key', 'value', callback)

Rather than:

var db = level()

db.open(function (err) {
  if (err) throw err
  db.put('key', 'value', callback)

TBD: whether this also includes methods like isOpen() and isClosed().

openCallback (boolean)

Does the db constructor take a callback?

var db = level(.., callback)

To the same effect as:

var db = level()
db.open(.., callback)

createIfMissing, errorIfExists (boolean)

Does db.open(options, ..) support these (leveldown) options?

promises (boolean)

Do all db methods (that don't otherwise have a return value) support promises, in addition to callbacks? Such that, when a callback argument is omitted, a promise is returned:

db.put('key', 'value', callback)
await db.put('key', 'value')

Note: iterators are currently exonerated because they, at the time of writing, don't support promises anywhere.

streams (boolean)

Does db have the methods createReadStream, createKeyStream and createValueStream, following the API currently documented in levelup?

encodings (boolean)

Do all relevant db methods take keyEncoding and valueEncoding options?

TBD: what this means for *asBuffer options.

additionalMethods (object)

In the form of:

  foo: true,
  bar: true

Which says the db has two methods, foo and bar, that are not part of the abstract-leveldown interface. It might be used like so:

if (db.supports.additionalMethods.foo) {

For future extensibility, the properties of additionalMethods should be taken as truthy rather than strictly typed booleans. We may add additional metadata (see #1).


With npm do:

npm install level-supports


Level/supports is an OPEN Open Source Project. This means that:

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