Possible for typescript type to have a required field, but be optional on "constructor"?-CodePudding

Attempting to have a Type with required fields, as each field is necessary, but default one of the parameters so I don't have to type it every time. For example:

export type Notification = {
  title: string
  message: string
  category: 'good' | 'bad'
}

const notifications: Notification[] = []

export const notify = (notification) => {
  notifications.push(notification)
}

So for the psuedo-constructor, at first it seems having Notification would be a good type for the input.

export const notify = (notification: Notification) => {
  notifications.push(notification)
}

But, what if the category will be good the vast majority of the time? Then I would want to make the category key optional on the function, and default it to good. However, due to the typing, category is required. How do I get around this?

I could create a new type:

export type NotifyInput = {
  title: string
  message: string
  category?: 'good' | 'bad'
}

export const notify = (notification: NotifyInput) => {
  notifications.push({
    ...notification,
    category: notification.category ?? 'good'
  })
}

but this isn't DRY at all, and I don't want to have to change multiple spots in the same file. Since i'm exporting the function, I can use it anywhere, so I don't want to have to export both types as well. I could also just inline the type on the function, but it has the same problems as before.

Sadly, I have to do one of the two sub-optimal options or my typescript compiler complains about notification being type any OR not containing category.

There has to be a better way than this. What is it?

CodePudding user response：

How about something like this? You specify the input type with an optional category and then export the Notification type with all properties required. Here's a working playground.

type NotificationInput = {
  title: string
  message: string
  category?: 'good' | 'bad'
}

export type Notification = Required<NotificationInput>;

const notifications: Notification[] = []

export const notify = (notification: NotificationInput) => {
  notifications.push({
    ...notification,
    category: notification.category || 'good'
  })
}