Derivation using the existing value

[ghost]

Hello! 👋

I'm trying to base the value in the derived factory on the value that was built by the original factory, but looks like this isn't possible with withDerivation or withDerivation1? (I attached an example below)

Is there any other way to achieve this? If not, would you consider adding this functionality into the library?

Example

import * as Factory from "factory.ts";

const userFactory = Factory.makeFactory({
  firstName: "Peter",
  lastName: "Parker"
});

// Doesn't work - firstName is an empty object
const userWithMiddleNameFactory = userFactory.withDerivation('firstName', (user) => {
  return user.firstName + " Benjamin"
});

// Also doesn't work
const altUserWithMiddleNameFactory = userFactory.withDerivation1(['firstName'], 'firstName', (firstName) => {
  return firstName + " Benjamin"
});

// Works for different fields
const userWithSameNamesFactory = userFactory.withDerivation('firstName', (user) => {
  return user.lastName
});

console.table({
  userWithMiddleNameFactory: userWithMiddleNameFactory.build(),
  altUserWithMiddleNameFactory: altUserWithMiddleNameFactory.build(),
  userWithSameNamesFactory: userWithSameNamesFactory.build()
});

which prints:

┌──────────────────────────────┬────────────────────────────┬──────────┐
│           (index)            │         firstName          │ lastName │
├──────────────────────────────┼────────────────────────────┼──────────┤
│  userWithMiddleNameFactory   │ '[object Object] Benjamin' │ 'Parker' │
│ altUserWithMiddleNameFactory │ '[object Object] Benjamin' │ 'Parker' │
│   userWithSameNamesFactory   │          'Parker'          │ 'Parker' │
└──────────────────────────────┴────────────────────────────┴──────────┘

p.s. This library has been super useful, thanks a lot for making it! 👏

[willryan]

@RussianCryptid should be fixed with #77 / v 1.4.0. (Use new function withSelfDerviation().)

[ghost]

@willryan Thank you! Just checked on the example and it's working.