Implement has() and hasMany()

Adds two methods:

```js
await db.put('love', 'u')
await db.has('love') // true
await db.hasMany(['love', 'hate']) // [true, false]
```

Ref: Level/community#142
Category: addition

README.md

@@ -141,6 +141,41 @@ Get multiple values from the database by an array of `keys`. The optional `optio
 
 Returns a promise for an array of values with the same order as `keys`. If a key was not found, the relevant value will be `undefined`.
 
+### `db.has(key[, options])`
+
+Check if the given `key` exists in the database. Returns a promise for a boolean. The optional `options` object may contain:
+
+- `keyEncoding`: custom key encoding for this operation, used to encode the `key`.
+- `snapshot`: explicit [snapshot](#snapshot--dbsnapshotoptions) to read from. If no `snapshot` is provided and `db.supports.implicitSnapshots` is true, the database will create its own internal snapshot for this operation.
+
+Use `has()` wisely and avoid the following pattern which has a race condition:
+
+```js
+if (await db.has('example')) {
+  const value = await db.get('example')
+  console.log(value)
+}
+```
+
+Instead do:
+
+```js
+const value = await db.get('example')
+
+if (value !== undefined) {
+  console.log(value)
+}
+```
+
+### `db.hasMany(keys[, options])`
+
+Check the existence of multiple `keys` given as an array. The optional `options` object may contain:
+
+- `keyEncoding`: custom key encoding for this operation, used to encode the `keys`.
+- `snapshot`: explicit [snapshot](#snapshot--dbsnapshotoptions) to read from. If no `snapshot` is provided and `db.supports.implicitSnapshots` is true, the database will create its own internal snapshot for this operation.
+
+Returns a promise for an array of booleans with the same order as `keys`.
+
 ### `db.put(key, value[, options])`
 
 Add a new entry or overwrite an existing entry. The optional `options` object may contain:

abstract-level.js

@@ -449,6 +449,117 @@ class AbstractLevel extends EventEmitter {
     return new Array(keys.length).fill(undefined)
   }
 
+  async has (key, options) {
+    options = getOptions(options, this[kDefaultOptions].key)
+
+    if (this[kStatus] === 'opening') {
+      return this.deferAsync(() => this.has(key, options))
+    }
+
+    assertOpen(this)
+
+    // TODO (next major): change this to an assert
+    const err = this._checkKey(key)
+    if (err) throw err
+
+    const snapshot = options.snapshot != null ? options.snapshot : null
+    const keyEncoding = this.keyEncoding(options.keyEncoding)
+    const keyFormat = keyEncoding.format
+
+    // Forward encoding options to the underlying store
+    if (options === this[kDefaultOptions].key) {
+      // Avoid Object.assign() for default options
+      options = this[kDefaultOptions].keyFormat
+    } else if (options.keyEncoding !== keyFormat) {
+      // Avoid spread operator because of https://bugs.chromium.org/p/chromium/issues/detail?id=1204540
+      options = Object.assign({}, options, { keyEncoding: keyFormat })
+    }
+
+    const encodedKey = keyEncoding.encode(key)
+    const mappedKey = this.prefixKey(encodedKey, keyFormat, true)
+
+    // Keep snapshot open during operation
+    if (snapshot !== null) {
+      snapshot.ref()
+    }
+
+    try {
+      return this._has(mappedKey, options)
+    } finally {
+      // Release snapshot
+      if (snapshot !== null) {
+        snapshot.unref()
+      }
+    }
+  }
+
+  async _has (key, options) {
+    throw new ModuleError('Database does not support has()', {
+      code: 'LEVEL_NOT_SUPPORTED'
+    })
+  }
+
+  async hasMany (keys, options) {
+    options = getOptions(options, this[kDefaultOptions].entry)
+
+    if (this[kStatus] === 'opening') {
+      return this.deferAsync(() => this.hasMany(keys, options))
+    }
+
+    assertOpen(this)
+
+    if (!Array.isArray(keys)) {
+      throw new TypeError("The first argument 'keys' must be an array")
+    }
+
+    if (keys.length === 0) {
+      return []
+    }
+
+    const snapshot = options.snapshot != null ? options.snapshot : null
+    const keyEncoding = this.keyEncoding(options.keyEncoding)
+    const keyFormat = keyEncoding.format
+
+    // Forward encoding options to the underlying store
+    if (options === this[kDefaultOptions].key) {
+      // Avoid Object.assign() for default options
+      options = this[kDefaultOptions].keyFormat
+    } else if (options.keyEncoding !== keyFormat) {
+      // Avoid spread operator because of https://bugs.chromium.org/p/chromium/issues/detail?id=1204540
+      options = Object.assign({}, options, { keyEncoding: keyFormat })
+    }
+
+    const mappedKeys = new Array(keys.length)
+
+    for (let i = 0; i < keys.length; i++) {
+      const key = keys[i]
+      const err = this._checkKey(key)
+      if (err) throw err
+
+      mappedKeys[i] = this.prefixKey(keyEncoding.encode(key), keyFormat, true)
+    }
+
+    // Keep snapshot open during operation
+    if (snapshot !== null) {
+      snapshot.ref()
+    }
+
+    try {
+      return this._hasMany(mappedKeys, options)
+    } finally {
+      // Release snapshot
+      if (snapshot !== null) {
+        snapshot.unref()
+      }
+    }
+  }
+
+  async _hasMany (keys, options) {
+    throw new ModuleError('Database does not support hasMany()', {
+      code: 'LEVEL_NOT_SUPPORTED'
+    })
+  }
+
   async put (key, value, options) {
     if (!this.hooks.prewrite.noop) {
       // Forward to batch() which will run the hook

lib/abstract-sublevel.js

@@ -146,6 +146,14 @@ module.exports = function ({ AbstractLevel }) {
       return this[kParent].getMany(keys, options)
     }
 
+    async _has (key, options) {
+      return this[kParent].has(key, options)
+    }
+
+    async _hasMany (keys, options) {
+      return this[kParent].hasMany(keys, options)
+    }
+
     async _del (key, options) {
       return this[kParent].del(key, options)
     }

test/has-many-test.js

@@ -0,0 +1,144 @@
+'use strict'
+
+const { illegalKeys } = require('./util')
+const traits = require('./traits')
+
+let db
+
+/**
+ * @param {import('tape')} test
+ */
+exports.setUp = function (test, testCommon) {
+  test('hasMany() setup', async function (t) {
+    db = testCommon.factory()
+    return db.open()
+  })
+}
+
+/**
+ * @param {import('tape')} test
+ */
+exports.args = function (test, testCommon) {
+  test('hasMany() requires an array argument', function (t) {
+    t.plan(6)
+
+    db.hasMany().catch(function (err) {
+      t.is(err && err.name, 'TypeError')
+      t.is(err && err.message, "The first argument 'keys' must be an array")
+    })
+
+    db.hasMany('foo').catch(function (err) {
+      t.is(err && err.name, 'TypeError')
+      t.is(err && err.message, "The first argument 'keys' must be an array")
+    })
+
+    db.hasMany('foo', {}).catch(function (err) {
+      t.is(err && err.name, 'TypeError')
+      t.is(err && err.message, "The first argument 'keys' must be an array")
+    })
+  })
+
+  test('hasMany() with illegal keys', function (t) {
+    t.plan(illegalKeys.length * 4)
+
+    for (const { name, key } of illegalKeys) {
+      db.hasMany([key]).catch(function (err) {
+        t.ok(err instanceof Error, name + ' - is Error')
+        t.is(err.code, 'LEVEL_INVALID_KEY', name + ' - correct error code')
+      })
+
+      db.hasMany(['valid', key]).catch(function (err) {
+        t.ok(err instanceof Error, name + ' - is Error (second key)')
+        t.is(err.code, 'LEVEL_INVALID_KEY', name + ' - correct error code (second key)')
+      })
+    }
+  })
+}
+
+/**
+ * @param {import('tape')} test
+ */
+exports.hasMany = function (test, testCommon) {
+  test('simple hasMany()', async function (t) {
+    await db.put('foo', 'bar')
+
+    t.same(await db.hasMany(['foo']), [true])
+    t.same(await db.hasMany(['foo'], {}), [true]) // same but with {}
+    t.same(await db.hasMany(['beep']), [false])
+
+    await db.put('beep', 'boop')
+
+    t.same(await db.hasMany(['beep']), [true])
+    t.same(await db.hasMany(['foo', 'beep']), [true, true])
+    t.same(await db.hasMany(['aaa', 'beep']), [false, true])
+    t.same(await db.hasMany(['beep', 'aaa']), [true, false], 'maintains order of input keys')
+  })
+
+  test('empty hasMany()', async function (t) {
+    t.same(await db.hasMany([]), [])
+
+    const encodings = Object.keys(db.supports.encodings)
+      .filter(k => db.supports.encodings[k])
+
+    for (const valueEncoding of encodings) {
+      t.same(await db.hasMany([], { valueEncoding }), [])
+    }
+  })
+
+  test('simultaneous hasMany()', async function (t) {
+    t.plan(20)
+
+    await db.put('hello', 'world')
+    const promises = []
+
+    for (let i = 0; i < 10; ++i) {
+      promises.push(db.hasMany(['hello']).then(function (values) {
+        t.same(values, [true])
+      }))
+    }
+
+    for (let i = 0; i < 10; ++i) {
+      promises.push(db.hasMany(['non-existent']).then(function (values) {
+        t.same(values, [false])
+      }))
+    }
+
+    return Promise.all(promises)
+  })
+
+  traits.open('hasMany()', testCommon, async function (t, db) {
+    t.same(await db.hasMany(['foo']), [false])
+  })
+
+  traits.closed('hasMany()', testCommon, async function (t, db) {
+    return db.hasMany(['foo'])
+  })
+
+  // Also test empty array because it has a fast-path
+  traits.open('hasMany() with empty array', testCommon, async function (t, db) {
+    t.same(await db.hasMany([]), [])
+  })
+
+  traits.closed('hasMany() with empty array', testCommon, async function (t, db) {
+    return db.hasMany([])
+  })
+}
+
+/**
+ * @param {import('tape')} test
+ */
+exports.tearDown = function (test, testCommon) {
+  test('hasMany() teardown', async function (t) {
+    return db.close()
+  })
+}
+
+/**
+ * @param {import('tape')} test
+ */
+exports.all = function (test, testCommon) {
+  exports.setUp(test, testCommon)
+  exports.args(test, testCommon)
+  exports.hasMany(test, testCommon)
+  exports.tearDown(test, testCommon)
+}