interface FindUserEmailProps {
   readonly email: string
}

interface FindUserIdProps {
   readonly id: string
}

type FindUserProps = FindUserEmailProps | FindUserIdProps

export const findUserByEmail = async ({ email }: FindUserProps): Promise<IUser> => {
   const user = await User.findOne({ email })
   if (!user) {
      throw new Error('User not found')
   }
   return user
}

At email property I got TS2339: Property 'email' does not exist on type 'FindUserProps' Why is that?

CodePudding user response：

That's because FindUserProps can be one of FindUserEmailProps or FindUserIdProps but it's not both (that would be FindUserEmailProps & FindUserIdProps). That means that TypeScript doesn't know which one it is until you assert it.

Your function has to take a FindUserProps and needs to add your own type guard to let TypeScript know whether it's a FindUserEmailProps or FindUserIdProps before you can extract an email property.

// Custom type guard that lets TypeScript know whether your
// object is a FindUserEmailProps
function isFindUserEmailProps(obj: FindUserProps): obj is FindUserEmailProps {
  return "email" in obj;
}

export const findUserByEmail = async (userProps: FindUserProps): Promise<IUser> => {
   // You have to make sure it's a FindUserEmailProps
   if (!isFindUserEmailProps(userProps)) {
      // Since you are just throwing an error, you may as well
      // change your function to only accept a FindUserEmailProps
      throw new Error("Invalid userProps to find by email");
   }
   const {email} = userProps;
   const user = await User.findOne({ email })
   if (!user) {
      throw new Error('User not found')
   }
   return user
}