Show/Hide Field Based on Boolean in Sanity - How Does It Work?

Great question! Your code is working perfectly, and you've actually discovered one of Sanity's most useful features - Conditional Fields. Let me break down exactly how it works:

How Your Code Works

{
  name: "flag",
  title: "Show or Hide",
  type: "boolean",
},
{
  name: "entry",
  type: "string",
  title: "Entry",
  hidden: ({document}) => !document.flag
}

The hidden property accepts a callback function that determines whether the field should be hidden. This function:

1. Receives a parameter object with {document} - this gives you access to the entire document being edited
2. Returns a boolean - true means hide the field, false means show it
3. Evaluates !document.flag - the ! (not operator) flips the boolean value:
   • When flag is true → !document.flag is false → field is shown
   • When flag is false → !document.flag is true → field is hidden

So your "entry" field appears when the checkbox is checked, and disappears when unchecked!

Other Useful Patterns

The callback function gives you access to several parameters you can use:

hidden: ({parent, document, value, currentUser}) => {
  // parent - the parent object (useful in nested fields)
  // document - the entire document
  // value - the current field's value
  // currentUser - info about who's editing
}

Example with multiple conditions:

{
  name: "specialContent",
  type: "text",
  hidden: ({document}) => document.contentType !== 'premium' || !document.isPublished
}

Example using parent (for fields inside objects):

{
  name: 'videoUrl',
  type: 'url',
  hidden: ({parent}) => parent?.mediaType !== 'video'
}

Pro Tip: Conditional Validation

If you ever need a field to be required only when visible, combine conditional fields with conditional validation:

{
  name: "entry",
  type: "string",
  hidden: ({document}) => !document.flag,
  validation: (Rule) => Rule.custom((value, context) => {
    if (context.document?.flag && !value) {
      return 'Entry is required when flag is checked'
    }
    return true
  })
}

Your solution is actually the recommended approach - clean, simple, and exactly how Sanity intended conditional fields to work. Welcome to Sanity, and happy building! 🎉