How to declare custom types in Sanity schema?

No worries, this is a common question when starting with Sanity! Creating custom types is actually straightforward once you understand the pattern.

In Sanity, you define custom types in your schema files using the defineType helper function. For the Personal Website template's "timeline" and "milestone" types, here's how it works:

Basic structure for a custom document type:

import { defineType, defineField } from 'sanity'

export const timeline = defineType({
  name: 'timeline',
  type: 'document',
  title: 'Timeline',
  fields: [
    defineField({
      name: 'title',
      type: 'string',
      title: 'Title',
    }),
    // more fields...
  ],
})

For a custom object type (like milestone):

export const milestone = defineType({
  name: 'milestone',
  type: 'object',  // Note: 'object' not 'document'
  title: 'Milestone',
  fields: [
    defineField({
      name: 'description',
      type: 'string',
      title: 'Description',
    }),
    // more fields...
  ],
})

Key differences:

• Document types (type: 'document') are top-level content that appear in your Studio and get their own IDs - these are standalone pieces of content
• Object types (type: 'object') are reusable structures that exist within documents - like a milestone inside a timeline

Then register them in your schema:

In your sanity.config.ts (or schema/index.ts):

import { timeline } from './schemas/timeline'
import { milestone } from './schemas/milestone'

export default defineConfig({
  // ... other config
  schema: {
    types: [timeline, milestone],
  },
})

Common errors to watch for:

• Not importing defineType and defineField from 'sanity'
• Forgetting to register your types in the schema configuration
• Using type: 'document' when you need type: 'object' (or vice versa)
• Name conflicts - make sure your name field is unique

The defineType function gives you TypeScript autocomplete and helps catch errors early, so always use it even though it's technically optional! If you're still getting errors, double-check that you've registered your types in the schema array and that all your imports are correct.