question: How to correctly use plainToInstance with Set<T>?

[sorashi]

I was trying to use a Set, which should be supported according to readme. Minimal example:

class Person {
    @Type(() => Number)
    public numbers: Set<number> = new Set()
}

const person = new Person()
person.numbers = new Set([1,2,3,4,5])
const plain = instanceToPlain(person)
const instance = plainToInstance(Person, plain)

console.log(person)
console.log(plain)
console.log(instance)

The problem:
Actual output:

Person { numbers: Set { 1, 2, 3, 4, 5 } }
{ numbers: [ 1, 2, 3, 4, 5 ] }
Person { numbers: [ 1, 2, 3, 4, 5 ] }

Expected output:

Person { numbers: Set { 1, 2, 3, 4, 5 } }
{ numbers: [ 1, 2, 3, 4, 5 ] }
Person { numbers: Set { 1, 2, 3, 4, 5 } }

How do I use it correctly?

[sorashi]

Solved using this

class Person {
    @Transform(value => new Set(value.value))
    public numbers: Set<number> = new Set()
}

But note that readme specifies this should work, which it doesn't

class-transformer/README.md

Lines 736 to 737 in 05b0151

	@Type(() => Skill)
	skills: Set<Skill>;

What is the problem here?

[ghost]

I just ran into this issue as well. I actually checked out the master codebase and ran this test, which passed.

  it('Creates a Set', () => {
    class TestSet {
      @Type(() => Set)
      test: Set<string>;

      constructor(data?: any) {
        Object.assign(this, plainToInstance(this.constructor as any, data))
      }
    }
    const record = new TestSet({
      test: ['a']
    })
    console.log(record.test.add('b'))
  })

I then did the exact same thing in my project using the same version (0.5.1), but received an error about add not being a function. I tried to trace plainToInstance, and I saw that it was accessing the CJS libs, so I'm not sure if that has something to do with it, but it's the only difference I could spot.

Regardless, @sorashi, your fix worked for me, thank you!

[diffy0712]

Hello,

could you provide some details on your environment please? The best would be if you could create a demo environment (repo, or stackblitz or something like that), which would help to reproduce the error.

Thanks.

[sorashi]

@diffy0712 Here you go https://github.com/sorashi/class-transformer-1446-repro

I observed this a year ago, so I tried replicating my environment back then. It's Vite+React+TypeScript app. The bug can be observed in the browser console. Instructions are in the readme in the repo. The code that uses class-transformer is in App.tsx.

[diffy0712]

Thank you, I will take a look!