TS2339: Property 'email' does not exist on type 'FindUserProps'

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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?

typescript express types
1 Answers

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
}
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