TypeScript - Handling Mapped Types with Nested Objects and Conditional Properties
I'm writing unit tests and I'm having a hard time understanding I'm working with a scenario when trying to create a mapped type that handles nested objects with conditional properties in TypeScript... I have a function that processes a configuration object for a form where each field can have different validation rules. The structure of the configuration is as follows: ```typescript interface FieldConfig { label: string; required: boolean; validation?: { [key: string]: any }; } interface FormConfig { [key: string]: FieldConfig; } const formConfig: FormConfig = { username: { label: 'Username', required: true }, email: { label: 'Email', required: true, validation: { email: true } }, age: { label: 'Age', required: false } }; ``` I'm trying to derive a type that ensures that if a field has validation rules, it must also have the `required` property set to `true`. However, when I attempt to create a mapped type like this: ```typescript type ValidatedFields<T> = { [K in keyof T]: T[K] extends { validation: any } ? (T[K] & { required: true }) : T[K]; }; ``` I get a TypeScript behavior: `Type 'FieldConfig' is not assignable to type 'ValidatedFields<FormConfig>'`. I’ve tried several variations of this type definition, but I keep running into issues with the conditional types not aligning correctly. I've also ensured that I’m using TypeScript version 4.5.2, which should support the features I’m attempting to use. My goal is to ensure type safety when these configurations are processed so that any misuse is caught at compile time. Can anyone help clarify what I might be doing wrong or suggest an alternative approach to achieve my goal? Has anyone else encountered this? Is there a simpler solution I'm overlooking?