TypeScript: exactOptionalPropertyTypes

TypeScript handles optional and undefined differently - exactOptionalPropertyTypes will help catch scenarios where undefined should be typed explicitly.

When turned off:

type User = {
  email?: string;
}

const user: User = { email: undefined }
// Valid

When turned on:

type User = {
  email?: string;
}

const user: User = { email: undefined }
    // Type '{ email: undefined; }' is not assignable to type 'User' with 'exactOptionalPropertyTypes: true'.
    // Consider adding 'undefined' to the types of the target's properties.

Read optional vs undefined | TkDodo's blog to learn the difference and how this can help.