Why Record type in TS with undefined as a value should have strictly defined fields?

Practically my question is related to a situation below:

I have a type for form values

interface FormValues {
  name: string
  description: string
  expirationDate: Date | null
}

and a type for errors that can be created if there are errors in a form

const errors: Record<keyof FormValues, string | undefined> = {
    name: undefined,
    description: undefined,
    expirationDate: undefined
}

however, the errors type works only if I define each fields as undefined, if I try to change it to an empty object, then TS responses with an error

const errors: Record<keyof FormValues, string | undefined> = {}
/*
Type '{}' is missing the following properties 
from type 'Record<keyof FormValues, string | undefined>': name, description, expirationDate
*/

how to change the error type to reflect my need in having an object with the fields defined as a string or non-exist:

const validate = values => {
  const errors: ??? = {}
  if (!values.name) errors.name = 'Required'
  if (!values.description) errors.description = 'Required'
  if (!values.expirationDate) errors.expirationDate = 'Required'
  return errors
}

Take a look at this (your type) :

type errorType1 = Record<keyof FormValues, string | undefined>

wich is equivalent to

type errorType1 = {
    name: string | undefined;
    description: string | undefined;
    expirationDate: string | undefined;
}

Then look at

type errorType2 = Partial<Record<keyof FormValues, string>>

which is equivalent to

type errorType2 = {
    name?: string | undefined;
    description?: string | undefined;
    expirationDate?: string | undefined;
}

With error type 2, you are allowed to forget keys. With the first you can't.

What you need is probably :

const errors: Partial<Record<keyof FormValues, string>> = {}

You can use the type utility Partial<Type> to make the keys of a type optional:

const errors: Partial<Record<keyof FormValues, string>> = {};

Here's a complete example based on the code from your question:

TS Playground

const formValueKeys = [
  "name",
  "description",
  "expirationDate",
] as const satisfies readonly (keyof FormValues)[];

const validate = (values: FormValues) => {
  const errors: Partial<Record<keyof FormValues, string>> = {};

  for (const key of formValueKeys) {
    if (!values[key]) errors[key] = "Required";
  }

  return errors;
};