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Schema validation rules to enforce image size or file type

By Simeon Griggs

Uses @sanity/asset-utils to validate an uploaded image by its ID

schema.ts

import { getExtension, getImageDimensions } from '@sanity/asset-utils'

// ...then in your schema
defineField({
  name: `image`,
  type: `image`,
  validation: (rule) =>
    rule.custom((value) => {
      if (!value) {
        return true
      }
      
      const filetype = getExtension(value.asset._ref)

      if (filetype !== 'jpg' && filetype !== 'png') {
        return 'Image must be a JPG or PNG'
      }
      
      const {width, height} = getImageDimensions(value.asset._ref)

      if (width < 1200 || height < 630) {
        return 'Image must be at least 1200x630 pixels'
      }

      return true
    }),
})

When assets are uploaded to Sanity's CDN they are given deterministic IDs which contain information about the original file's dimensions and file format.

The @sanity/asset-utils library contains a set of helpful functions to easily extract those details from an asset's ID.

In this snippet, we use those values to inform a custom validation rule for an Image field.

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