Add TypeScript Type Checks to RouteData fields

> Inspired by error: Conversion of type may be a mistake because neither type sufficiently overlaps with the other.

January 28, 2023

All the world's a type and we are all merely types

The Problem

RouteData is an interface defined as:

export interface RouteData {
    name: string;
    displayName?: string;
    fields?: {
        [name: string]: Field;
    };
    databaseName?: string;
    deviceId?: string;
    itemLanguage?: string;
    itemVersion?: number;
    layoutId?: string;
    templateId?: string;
    templateName?: string;
    placeholders: PlaceholdersData;
    itemId?: string;
}

Whereby this:

const foo = useSitecoreContext().sitecoreContext.route?.fields;

Corresponds to:

(property) RouteData.fields?: {
    [name: string]: Field<GenericFieldValue>;
} | undefined

In other words, RouteData.fields acts as a kind of arbitrary dictionary.

Now imagine that I have defined a custom type (the full type actually has 25+ fields):

export type Publication = {
  id?: string;
  url?: string;
  fields: {
    abstract: Field<string>;
    attachment: FileField;
  };
}

I'd like to be able to do this:

const foo = useSitecoreContext().sitecoreContext.route?.fields as Publication['fields'];

However, I get this IDE / build error:

Conversion of type '{ [name: string]: Field<GenericFieldValue>; } | undefined' to type '{ abstract: Field<string>; attachment: FileField; }' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
  Type '{ [name: string]: Field<GenericFieldValue>; }' is missing the following properties from type '{ abstract: Field<string>; attachment: FileField; }': abstract, attachment

If I do as the error suggests, the IDE and build gods are pleased:

const bar = (useSitecoreContext().sitecoreContext.route?.fields as unknown) as Publication['fields'];

Let's say you need a component that renders a header for a "blog" page. The header should display the title of the blog and the type of blog. Both fields are specified on the page item (and therefore the route). The blog type is a list field that is a reference to a custom type called BlogType. BlogType has a single field called Name. The header component should be able to render the title and the name of the blog type.

BlogHeader.tsx

import { Text, useSitecoreContext, Field } from '@sitecore-jss/sitecore-jss-nextjs';
import { Website } from 'lib/component-props/model';

type BlogHeaderRouteFields = Website.Project.Main.PageTypes.Blogs.Publication['fields'];

const BlogHeader = (): JSX.Element => {
  const { sitecoreContext } = useSitecoreContext();

  if (!sitecoreContext || !sitecoreContext?.route?.fields) {
    return <></>;
  }

  const blogHeaderRouteFields = (sitecoreContext.route.fields as unknown) as BlogHeaderRouteFields;

  return (
    <div>
      <h1 className="blog-header">
        <Text field={blogHeaderRouteFields.title as Field<string>} />
      </h1>
      {blogHeaderRouteFields.blogTypes?.value && (
        <div className="blog-type">
          <Text
            field={blogHeaderRouteFields.blogTypes.value[0]?.fields?.name as Field<string>}
            editable={false}
          />
        </div>
      )}
    </div>
  );
};

export default BlogHeader;

The Better Solution

sitecoreContext has no knowledge of what RouteData it has; specifically, fields. It defaults to Record<string, string | Field | Item | Item[]>, which means we need to assert what is in it when we use it.

The better solution to this would be to have a generic type for RouteData that allows us to specify the type of fields. If generics were supported we could do this:

const { sitecoreContext } = useSitecoreContext<MyRouteFields>(); 

This is cleaner and more less prone to errors because assertions don't need to be added whenever route fields are referenced. I am also not the first one to notice or ask for this:

https://github.com/Sitecore/jss/issues/1016

The OP describes the ask perfectly and offers a temporary solution until someone takes the precious time to submit a PR.

-MG