feat(core): stackable errors

allows for errors to be passed as the contructor argument

lib/index.js

@@ -1,16 +1,24 @@
 'use strict'
 
 module.exports = class ExtendableError extends Error {
-  constructor (message) {
-    super(message)
+  constructor (input) {
+    const isError = Object.prototype.toString.call(input) === '[object Error]' || input instanceof Error
 
-    this.name = this.constructor.name
+    const message = (isError ? input.message : input) || ''
+
+    super(message)
     this.message = message
+    this.name = this.constructor.name
+
+    // inherit properties
+    if (isError) {
+      Object.assign(this, input)
+    }
 
     if (typeof Error.captureStackTrace === 'function') {
       Error.captureStackTrace(this, this.constructor)
     } else {
-      this.stack = (new Error(message)).stack
+      this.stack = new Error(message).stack
     }
   }
 }

test/index.js

@@ -8,6 +8,19 @@ class SubTestError extends TestError {}
 
 const regex = /(\w|\/)+test\/index\.js:\d{2}:\d{2}/
 
+test('Empty ExtendableError', assert => {
+  assert.plan(5)
+
+  let err = new ExtendableError()
+
+  assert.type(err, ExtendableError)
+
+  assert.equal(err.name, 'ExtendableError')
+  assert.equal(err.message, '')
+  assert.match(err.stack, regex)
+  assert.equal(err.toString(), 'ExtendableError')
+})
+
 test('ExtendableError instance of Error', assert => {
   assert.plan(6)
 
@@ -53,6 +66,27 @@ test('SubTestError instance of TestError', assert => {
   assert.equal(err.toString(), 'SubTestError: error occurred')
 })
 
+test('Stacking Errors', assert => {
+  assert.plan(6)
+
+  let error = new Error('error occurred')
+  error.code = 10
+  error.foo = 'bar'
+
+  let err = new ExtendableError(error)
+
+  assert.type(err, ExtendableError)
+  assert.type(err, Error)
+
+  assert.equal(err.code, 10)
+  assert.equal(err.foo, 'bar')
+  assert.equal(err.name, 'ExtendableError')
+  assert.equal(err.toString(), 'ExtendableError: error occurred')
+
+  assert.end()
+})
+
+// must be last, breaks tap otherwise
 test('Manual captureStackTrace', assert => {
   assert.plan(6)