Tips for restricting keys when using a union as an indexer without demanding all of them

Trying to create a type that only allows certain keys from a union using the key in statement has resulted in all keys being required. How can I define this type so that not all values have to be present?

const widgetOptions = ['option1', 'option2', 'option3'] as const
export type WidgetOption = (typeof widgetOptions)[number]

export type WidgetData = {
  [key: string]: {
    [key in WidgetOption]?: number
  }
}

const widgets: WidgetData = {
  'item1': {
    option1: 10,
    // no error here without providing option2 or option3
  },
  'item2': {
    option1: 20,
    option2: 30
  }
} satisfies WidgetData

Answer №1

Using mapped types allows you to apply the readonly modifier to make properties read-only in the new type, as well as adding the ? symbol to make them optional.

To achieve this, your code should look like this:

export type WidgetSummaryData = {
  [key: string]: {
    [key in WidgetCondition]?: number
  }
}

This setup makes all properties optional in the new type.

For more information on modifying mapped types, refer to the official documentation.

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