Make Multer errors inherit from MulterError (#641)

* Add "isMulter" property set to true to an error instance

* Remove "isMulter" property from an error instance

* Add ./lib/multer-error.js file

* Update ./lib/make-error.js file

* Require and export MulterError class in ./index.js file

* Fix typo in ./lib/make-error.js file

* Add test case to ./test/error-handling.js file

* Update README.md

* Add `if` statement for the `field` property

* Remove ./lib/make-error.js file and replace all the `makeError` invocations with `new MulterError` class

Author: turakvlad

README.md

@@ -266,23 +266,25 @@ function fileFilter (req, file, cb) {
 
 ## Error handling
 
-When encountering an error, multer will delegate the error to express. You can
+When encountering an error, Multer will delegate the error to Express. You can
 display a nice error page using [the standard express way](http://expressjs.com/guide/error-handling.html).
 
-If you want to catch errors specifically from multer, you can call the
-middleware function by yourself.
+If you want to catch errors specifically from Multer, you can call the
+middleware function by yourself. Also, if you want to catch only [the Multer errors](https://github.com/expressjs/multer/blob/master/lib/make-error.js#L1-L9), you can use the `MulterError` class that is attached to the `multer` object itself (e.g. `err instanceof multer.MulterError`).
 
 ```javascript
+var multer = require('multer')
 var upload = multer().single('avatar')
 
 app.post('/profile', function (req, res) {
   upload(req, res, function (err) {
-    if (err) {
-      // An error occurred when uploading
-      return
+    if (err instanceof multer.MulterError) {
+      // A Multer error occurred when uploading.
+    } else {
+      // An unknown error occurred when uploading.
     }
 
-    // Everything went fine
+    // Everything went fine.
   })
 })
 ```

index.js

@@ -1,8 +1,8 @@
-var makeError = require('./lib/make-error')
 var makeMiddleware = require('./lib/make-middleware')
 
 var diskStorage = require('./storage/disk')
 var memoryStorage = require('./storage/memory')
+var MulterError = require('./lib/multer-error')
 
 function allowAll (req, file, cb) {
   cb(null, true)
@@ -37,7 +37,7 @@ Multer.prototype._makeMiddleware = function (fields, fileStrategy) {
 
     function wrappedFileFilter (req, file, cb) {
       if ((filesLeft[file.fieldname] || 0) <= 0) {
-        return cb(makeError('LIMIT_UNEXPECTED_FILE', file.fieldname))
+        return cb(new MulterError('LIMIT_UNEXPECTED_FILE', file.fieldname))
       }
 
       filesLeft[file.fieldname] -= 1
@@ -101,3 +101,4 @@ function multer (options) {
 module.exports = multer
 module.exports.diskStorage = diskStorage
 module.exports.memoryStorage = memoryStorage
+module.exports.MulterError = MulterError

lib/make-middleware.js

@@ -5,7 +5,7 @@ var onFinished = require('on-finished')
 var appendField = require('append-field')
 
 var Counter = require('./counter')
-var makeError = require('./make-error')
+var MulterError = require('./multer-error')
 var FileAppender = require('./file-appender')
 var removeUploadedFiles = require('./remove-uploaded-files')
 
@@ -76,7 +76,7 @@ function makeMiddleware (setup) {
     }
 
     function abortWithCode (code, optionalField) {
-      abortWithError(makeError(code, optionalField))
+      abortWithError(new MulterError(code, optionalField))
     }
 
     // handle text field data

lib/make-error.js renamed to lib/multer-error.js

@@ -1,3 +1,5 @@
+var util = require('util')
+
 var errorMessages = {
   'LIMIT_PART_COUNT': 'Too many parts',
   'LIMIT_FILE_SIZE': 'File too large',
@@ -8,11 +10,14 @@ var errorMessages = {
   'LIMIT_UNEXPECTED_FILE': 'Unexpected field'
 }
 
-function makeError (code, optionalField) {
-  var err = new Error(errorMessages[code])
-  err.code = code
-  if (optionalField) err.field = optionalField
-  return err
+function MulterError (code, field) {
+  Error.captureStackTrace(this, this.constructor)
+  this.name = this.constructor.name
+  this.message = errorMessages[code]
+  this.code = code
+  if (field) this.field = field
 }
 
-module.exports = makeError
+util.inherits(MulterError, Error)
+
+module.exports = MulterError

test/error-handling.js

@@ -14,6 +14,23 @@ function withLimits (limits, fields) {
 }
 
 describe('Error Handling', function () {
+  it('should be an instance of both `Error` and `MulterError` classes in case of the Multer\'s error', function (done) {
+    var form = new FormData()
+    var storage = multer.diskStorage({ destination: os.tmpdir() })
+    var upload = multer({storage: storage}).fields([
+      { name: 'small0', maxCount: 1 }
+    ])
+
+    form.append('small0', util.file('small0.dat'))
+    form.append('small0', util.file('small0.dat'))
+
+    util.submitForm(upload, form, function (err, req) {
+      assert.equal(err instanceof Error, true)
+      assert.equal(err instanceof multer.MulterError, true)
+      done()
+    })
+  })
+
   it('should respect parts limit', function (done) {
     var form = new FormData()
     var parser = withLimits({ parts: 1 }, [