With exactOptionalPropertyTypes enabled, TypeScript has stricter rules around what you can set an optional property to. For example, this interface declares that there is a property which can be one of two strings: 'dark' or 'light' or it should not be in the object.

interface UserDefaults {
  // The absence of a value represents 'system'
  colorThemeOverride?: "dark" | "light";
}

// Without this flag enabled, there are three values which you can set `colorThemeOverride` to be: "dark", "light" and `undefined`. Setting the value to `undefined` will allow most JavaScript runtime checks for the existence to fail, which is effectively falsy. However, there's a fuzziness to this behavior. The definition for `colorThemeOverride` _doesn't_ include `undefined`. The flag `exactOptionalPropertyTypes` makes TypeScript accurately match the definition provided as an optional property.

declare function getUserSettings(): UserDefaults;

const settings = getUserSettings();
settings.colorThemeOverride = "dark";
settings.colorThemeOverride = "light";

// But not:
settings.colorThemeOverride = undefined;

// Prior to the flag `exactOptionalPropertyTypes`, this was not possible.