Transforming objects created by Object.create(null) produce unexpected results.

[blaugold]

When transforming a plain object created by Object.create(null) plainToClass does not return a new instance of the given constructor. Instead, it returns the original object:

import { plainToClass } from 'class-transformer'

class A {

  a: string

}

function initObject(o: any): any {
  o.a = 'A'
  return o
}


const objectWithoutPrototype = Object.create(null)
const object                 = {}

const instanceFromObject                 = plainToClass(A, initObject(object))
const instanceFromObjectWithoutPrototype = plainToClass(A, initObject(objectWithoutPrototype))

printInstance('instanceFromObject', instanceFromObject)
printInstance('instanceFromObjectWithoutPrototype', instanceFromObjectWithoutPrototype)

console.log('instanceFromObjectWithoutPrototype === objectWithoutPrototype',
  instanceFromObjectWithoutPrototype === objectWithoutPrototype)

function printInstance(name: string, inst: any) {
  const prototype   = Object.getPrototypeOf(inst)
  const constructor = prototype && prototype.constructor

  console.log(`${name}:`)
  console.log('prototype', prototype)
  console.log('constructor', constructor && constructor.name)
  console.log('instance', inst)
  console.log('')
}

When exectued:

instanceFromObject:
prototype A {}
constructor A
instance A { a: 'A' }

instanceFromObjectWithoutPrototype:
prototype null
constructor null
instance { a: 'A' }

instanceFromObjectWithoutPrototype === objectWithoutPrototype true

"class-transformer": "^0.1.8"

[bjyoungblood]

👍 here. Apparently the Express.js team decided that it's best for user-supplied objects to have a null prototype (see links below). Unfortunately, this means that instanceof Object checks will fail on these objects.

expressjs/multer#187
expressjs/discussions#25
expressjs/multer#318

The culprit is here.

The simplest fix would be to add || typeof value === 'object' to the condition.

As for the potential for future debugging, my personal preference would be to throw an error from the else case of TransformOperationExecutor::transform instead of returning the original value, especially since returning the original value violates the return type contract defined by plainToClass.

[blaugold]

I stumbled across this issue because GraphQL does the same.

[NoNameProvided]

Thanks for the heads up @bjyoungblood!

This looks serious. Can you propose a PR with your minimal fix for this?

[NoNameProvided]

This has been fixed in 0cd39ed and will be included in the next release.

[NoNameProvided]

Just published 0.1.9, please test it and report if you still encounter this issue. Thanks!

[github-actions]

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