Promises function in Typescript “type void is not assignable …”

Question

I have just started using Typescript. One problem I can't seem to solve without cheating the Typescript system and using any type is the following:

I have a function that returns a promise:

  saveExpirableToken(tokenField: string, tokenExpiresField: string, tokenExpiresAt: Date) {
    return new Promise<UserDocument>((resolve, reject) => {
      this.generateResetToken((err: Error, resetToken: string) => {
        if (err) { reject(err) }

        this[tokenField] = resetToken
        this[tokenExpiresField] = tokenExpiresAt
        resolve(this.save())
      })
    })
  },

I call elsewhere the following

    let winningUser: UserDocument = await randomUser.saveExpirableToken(
      'wonEverythingToken',
      'wonEverythingTokenExpires',
      moment().add(1, 'year').toDate() // add one year as expiration to claiming prize
    )

The Typescript error I get is: Type 'void' is not assignable to type 'UserDocument'

What am I missing with my function declaration? And how do I handle Promises with Typescript such that I can tell Typescript that winningUser will in fact be of type UserDocument

** Edit **

  generateResetToken(callback: (err: Error | null, token?: string) => Promise<UserDocument>) {
    const byteSize = 16
    crypto.randomBytes(byteSize, (err, resetToken) => {
      if (err) { callback(err) }

      callback(null, resetToken.toString('hex'))
    })
  },

Answer 1

Please check out the type definition of this.save() method, it seems like it does not return UserDocument but instead returning void .

I've created a playground link and it shows that your other type definitions is working just fine: Link