Add support for transformers as async functions

Closes GH-35.

index.js

@@ -6,7 +6,6 @@ var bail = require('bail')
 var vfile = require('vfile')
 var trough = require('trough')
 var string = require('x-is-string')
-var func = require('x-is-function')
 var plain = require('is-plain-obj')
 
 /* Expose a frozen processor. */
@@ -123,7 +122,7 @@ function unified() {
 
       transformer = plugin.apply(processor, values.slice(1))
 
-      if (func(transformer)) {
+      if (typeof transformer === 'function') {
         transformers.use(transformer)
       }
     }
@@ -176,7 +175,7 @@ function unified() {
 
     if (value === null || value === undefined) {
       /* Empty */
-    } else if (func(value)) {
+    } else if (typeof value === 'function') {
       addPlugin.apply(null, arguments)
     } else if (typeof value === 'object') {
       if ('length' in value) {
@@ -203,7 +202,7 @@ function unified() {
     }
 
     function add(value) {
-      if (func(value)) {
+      if (typeof value === 'function') {
         addPlugin(value)
       } else if (typeof value === 'object') {
         if ('length' in value) {
@@ -287,7 +286,7 @@ function unified() {
     assertNode(node)
     freeze()
 
-    if (!cb && func(file)) {
+    if (!cb && typeof file === 'function') {
       cb = file
       file = null
     }
@@ -411,7 +410,7 @@ function unified() {
 
 /* Check if `func` is a constructor. */
 function newable(value) {
-  return func(value) && keys(value.prototype)
+  return typeof value === 'function' && keys(value.prototype)
 }
 
 /* Check if `value` is an object with keys. */
@@ -425,14 +424,14 @@ function keys(value) {
 
 /* Assert a parser is available. */
 function assertParser(name, Parser) {
-  if (!func(Parser)) {
+  if (typeof Parser !== 'function') {
     throw new Error('Cannot `' + name + '` without `Parser`')
   }
 }
 
 /* Assert a compiler is available. */
 function assertCompiler(name, Compiler) {
-  if (!func(Compiler)) {
+  if (typeof Compiler !== 'function') {
     throw new Error('Cannot `' + name + '` without `Compiler`')
   }
 }

package.json

@@ -29,7 +29,6 @@
     "is-plain-obj": "^1.1.0",
     "trough": "^1.0.0",
     "vfile": "^2.0.0",
-    "x-is-function": "^1.0.4",
     "x-is-string": "^0.1.0"
   },
   "devDependencies": {

test/async-function.js

@@ -0,0 +1,35 @@
+'use strict'
+
+var test = require('tape')
+var vfile = require('vfile')
+var unified = require('..')
+
+test('async function transformer () {}', function(t) {
+  var f
+  var n
+  var e
+
+  t.plan(5)
+
+  f = vfile('alpha')
+  n = {type: 'bravo'}
+  e = {type: 'charlie'}
+
+  unified()
+    .use(plugin)
+    .run(n, f, function(err, a, file) {
+      t.error(err, 'should’t fail')
+      t.equal(a, e, 'passes given tree to `done`')
+      t.equal(file, f, 'passes given file to `done`')
+    })
+
+  function plugin() {
+    return transformer
+  }
+
+  async function transformer(tree, file) {
+    t.equal(tree, n, 'passes correct tree to an async function')
+    t.equal(file, f, 'passes correct file to an async function')
+    return e
+  }
+})

test/index.js

@@ -10,3 +10,15 @@ require('./parse')
 require('./run')
 require('./stringify')
 require('./process')
+
+var asyncfunctions = false
+
+try {
+  eval('typeof async function() {}') // eslint-disable-line no-eval
+  asyncfunctions = true
+} catch (err) {}
+
+console.log('asyncfunctions: ', asyncfunctions)
+if (asyncfunctions) {
+  require('./async-function')
+}