Align nextTick behavior with abstract-leveldown

By using `queueMicrotask()` in browsers, for a consistent experience
regardless of bundler (and of runtime in the future, when we drop
node 10). Webpack no longer shims node core modules.

In theory bundlers can now skip shimming `process`, were it not for
our `readable-stream` dependency which uses `process.nextTick()` at
the time of writing and has more reason to keep doing so. In modules
where the risk of timing issues (due to not having a common microtask
scheduler between modules) is low, like here in `levelup`, I prefer
the idiomatic `queueMicroTask()` function.

Lastly, maybe streams are not here to stay. Async iterators are
coming to the level ecosystem. Or maybe we'll switch to `streamx`
which is planning to use `queueMicrotask()` as well.

lib/levelup.js

@@ -12,6 +12,9 @@ const catering = require('catering')
 const getCallback = require('./common').getCallback
 const getOptions = require('./common').getOptions
 
+// TODO: after we drop node 10, also use queueMicrotask() in node
+const nextTick = require('./next-tick')
+
 const WriteError = errors.WriteError
 const ReadError = errors.ReadError
 const NotFoundError = errors.NotFoundError
@@ -46,7 +49,7 @@ function LevelUP (db, options, callback) {
   if (!db || typeof db !== 'object') {
     error = new InitializationError('First argument must be an abstract-leveldown compliant store')
     if (typeof callback === 'function') {
-      return process.nextTick(callback, error)
+      return nextTick(callback, error)
     }
     throw error
   }
@@ -97,7 +100,7 @@ LevelUP.prototype.open = function (opts, callback) {
   }
 
   if (this.isOpen()) {
-    process.nextTick(callback, null, this)
+    nextTick(callback, null, this)
     return callback.promise
   }
 
@@ -132,7 +135,7 @@ LevelUP.prototype.close = function (callback) {
     this.emit('closing')
     this.db = new DeferredLevelDOWN(this._db)
   } else if (this.isClosed()) {
-    process.nextTick(callback)
+    nextTick(callback)
   } else if (this.db.status === 'closing') {
     this.once('closed', callback)
   } else if (this._isOpening()) {
@@ -299,7 +302,7 @@ LevelUP.prototype.type = 'levelup'
 
 function maybeError (db, callback) {
   if (!db._isOpening() && !db.isOpen()) {
-    process.nextTick(callback, new ReadError('Database is not open'))
+    nextTick(callback, new ReadError('Database is not open'))
     return true
   }
 }

lib/next-tick-browser.js

@@ -0,0 +1,11 @@
+'use strict'
+
+const queueMicrotask = require('queue-microtask')
+
+module.exports = function (fn, ...args) {
+  if (args.length === 0) {
+    queueMicrotask(fn)
+  } else {
+    queueMicrotask(() => fn(...args))
+  }
+}

lib/next-tick.js

@@ -0,0 +1,3 @@
+'use strict'
+
+module.exports = process.nextTick

package.json

@@ -4,13 +4,16 @@
   "description": "Fast & simple storage - a Node.js-style LevelDB wrapper",
   "license": "MIT",
   "main": "lib/levelup.js",
+  "browser": {
+    "./lib/next-tick.js": "./lib/next-tick-browser.js"
+  },
   "scripts": {
     "test": "standard && hallmark && (nyc -s node test/self.js | faucet) && nyc report",
     "coverage": "nyc report --reporter=text-lcov | coveralls",
     "test-browsers": "airtap --verbose test/self.js > airtap.log",
     "test-browsers-local": "airtap -p local test/self.js",
     "hallmark": "hallmark --fix",
-    "dependency-check": "dependency-check --no-dev .",
+    "dependency-check": "dependency-check --no-dev -i queue-microtask .",
     "prepublishOnly": "npm run dependency-check"
   },
   "files": [
@@ -25,7 +28,8 @@
     "deferred-leveldown": "~5.3.0",
     "level-errors": "^3.0.0",
     "level-iterator-stream": "^5.0.0",
-    "level-supports": "^2.0.0"
+    "level-supports": "^2.0.0",
+    "queue-microtask": "^1.2.3"
   },
   "devDependencies": {
     "after": "^0.8.2",

test/create-stream-vs-put-racecondition.js

@@ -1,4 +1,5 @@
 const levelup = require('../lib/levelup')
+const nextTick = require('../lib/next-tick')
 const memdown = require('memdown')
 const encdown = require('encoding-down')
 const after = require('after')
@@ -23,7 +24,7 @@ function makeTest (test, encode, deferredOpen, delayedPut) {
 
   test(name, function (t) {
     const db = encode ? levelup(encdown(memdown())) : levelup(memdown())
-    const delay = delayedPut ? process.nextTick : callFn
+    const delay = delayedPut ? nextTick : callFn
 
     run(t, db, !deferredOpen, delay)
   })

test/idempotent-test.js

@@ -1,4 +1,5 @@
 const levelup = require('../lib/levelup.js')
+const nextTick = require('../lib/next-tick')
 const memdown = require('memdown')
 const sinon = require('sinon')
 
@@ -18,7 +19,7 @@ module.exports = function (test, testCommon) {
 
       // close needs to be idempotent too.
       db.close()
-      process.nextTick(db.close.bind(db))
+      nextTick(db.close.bind(db))
     }
 
     const db = levelup(memdown(), function () {

test/init-test.js

@@ -1,4 +1,5 @@
 const levelup = require('../lib/levelup')
+const nextTick = require('../lib/next-tick')
 const memdown = require('memdown')
 
 module.exports = function (test, testCommon) {
@@ -51,7 +52,7 @@ module.exports = function (test, testCommon) {
 
     const mem = memdown()
     mem._open = function (opts, cb) {
-      process.nextTick(cb, new Error('from underlying store'))
+      nextTick(cb, new Error('from underlying store'))
     }
 
     levelup(mem, function (err) {
@@ -68,7 +69,7 @@ module.exports = function (test, testCommon) {
 
     const mem = memdown()
     mem._open = function (opts, cb) {
-      process.nextTick(cb, new Error('from underlying store'))
+      nextTick(cb, new Error('from underlying store'))
     }
 
     levelup(mem)

test/read-stream-test.js

@@ -3,6 +3,7 @@ const bigBlob = Array.apply(null, Array(1024 * 100)).map(function () { return 'a
 const discardable = require('./util/discardable')
 const readStreamContext = require('./util/rs-context')
 const rsFactory = require('./util/rs-factory')
+const nextTick = require('../lib/next-tick')
 
 module.exports = function (test, testCommon) {
   const createReadStream = rsFactory(testCommon)
@@ -523,7 +524,7 @@ module.exports = function (test, testCommon) {
       const rs = createReadStream(db)
         .on('close', done)
 
-      process.nextTick(function () {
+      nextTick(function () {
         rs.destroy()
       })
     })

test/self.js

@@ -50,6 +50,6 @@ suite({
 // Integration tests that can't use a generic testCommon.factory()
 require('./self/manifest-test')
 
-if (!process.browser) {
+if (typeof process === 'undefined' || !process.browser) {
   require('./browserify-test')(test)
 }

test/snapshot-test.js

@@ -3,6 +3,7 @@ const trickle = require('trickle')
 const discardable = require('./util/discardable')
 const readStreamContext = require('./util/rs-context')
 const rsFactory = require('./util/rs-factory')
+const nextTick = require('../lib/next-tick')
 
 module.exports = function (test, testCommon) {
   const createReadStream = rsFactory(testCommon)
@@ -28,7 +29,7 @@ module.exports = function (test, testCommon) {
           done()
         }, 0.05))
 
-        process.nextTick(function () {
+        nextTick(function () {
           // 3) Concoct and write new random data over the top of existing items.
           //    If we're not using a snapshot then then we'd expect the test
           //    to fail because it'll pick up these new values rather than the