What is the importance of specifying exact fields for Record type in TypeScript when initializing it with undefined?

My question pertains to a specific scenario:

I have defined a type for form values

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

and a type for potential errors that may occur in the form

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

However, the errors type only works when each field is explicitly defined as undefined. If I attempt to change it to an empty object, TypeScript throws 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 can I modify the error type to match my requirement of having an object with fields either defined as a string or non-existent:

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
}

Answer №1

To make the keys of a type optional, you can utilize the type utility called Partial<Type>:

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

Below is an example illustrating this concept using code similar to what you provided:

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;
};

Answer №2

Check out this example of (your type) :

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

which is the same as

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

Next, consider

type errorMessageType2 = Partial<Record<keyof FormValues, string>>

equivalent to

type errorMessageType2 = {
    name?: string | undefined;
    description?: string | undefined;
    expirationDate?: string | undefined;
}

With error type 2, you have the option to leave keys blank. With the first type, you can't.

You probably need :

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

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