How to use Typescript to define strongly typed email template parameters?

Question

I have a number of different email templates that each have different parameters.

I can define a type for each template:

type TemplateType = 'welcomeEmail' | 'referralEmail' | 'loginEmail'

And for each template I can define parameters:

interface WelcomeEmail {
  firstName: string;
  lastName: string;
  ...
}

I a can then define an EmailTemplate interface:

interface EmailTemplate {
  template: TemplateType;
  params: WelcomeEmail | ReferralEmail | LoginEmail;
}

Is there a way to type this so that when the template is 'welcomeEmail' the params type is WelcomeEmail ?

Answer 1

It can be done in recent versions of Typescript if you change your types around a bit. This works by sourcing the TemplateType and the EmailTemplate from the same EmailTemplates type. Typescript knows that members of TemplateType correspond to keys of EmailTemplates , and it can use that relationship to find the value types that are associated with a given TemplateType .

type EmailTemplates = {
  welcomeEmail: {
    firstName: string;
    lastName: string;
  };
  referralEmail: {
    referrer: string;
  };
}

type TemplateType = keyof EmailTemplates;

type EmailTemplate<T extends TemplateType> = {
  template: T;
  params: EmailTemplates[T]
}

const myEmail: EmailTemplate<'referralEmail'> = {
  template: 'referralEmail',
  params: { referrer: 'test' },
}

There may be some way to infer the TemplateType instead of having to pass it in as a generic type parameter, but I'm not sure how.

Answer 2

type TemplateType = 'welcomeEmail' | 'referralEmail' | 'loginEmail' interface WelcomeEmail { firstName: string; lastName: string; // ... } interface WelcomeEmailTemplate { template: 'welcomeEmail'; params: WelcomeEmail; } interface ReferralEmailTemplate { // ... } interface LoginEmailTemplate { // ... } type EmailTemplate = WelcomeEmailTemplate | ReferralEmailTemplate | LoginEmailTemplate const myEmail: EmailTemplate = { template: 'welcomeEmail', params: { // then type checking and hinting is here for WelcomeEmailTemplate }, }