ianstormtaylor/slate · error · Error

Cannot set the "then" property of the selection to a functio

Error message

Cannot set the "then" property of the selection to a function

What it means

Slate refuses to set a 'then' property on the selection when the value is a function, because a thenable selection would be treated as a Promise-like object. Returning such a selection from an async function would auto-await it and cause surprising behavior, so Slate guards against it explicitly.

Source

Thrown at packages/slate/src/interfaces/transforms/general.ts:328

        }

        const selection = { ...editor.selection }

        for (const key in newProperties) {
          if (NON_SETTABLE_SELECTION_PROPERTIES.includes(key)) {
            throw new Error(
              `Cannot set the "${key}" property of the selection!`
            )
          }

          const value = newProperties[<keyof Range>key]

          // Make sure we're not setting `then` to a function, since this will
          // cause the selection to be treated as a Promise-like object, which
          // can cause unexpected behaviour when returning the selection from
          // async functions.
          if (key === 'then' && typeof value === 'function') {
            throw new Error(
              'Cannot set the "then" property of the selection to a function'
            )
          }

          if (value == null) {
            if (key === 'anchor' || key === 'focus') {
              throw new Error(`Cannot remove the "${key}" selection property`)
            }

            delete selection[<keyof Range>key]
          } else {
            selection[<keyof Range>key] = value
          }
        }

        editor.selection = selection

        break

View on GitHub (pinned to 72a37c701e)

Solutions

1. Strip any 'then' key from selection properties before calling setSelection
2. Validate that no property value in the operation is a function before applying remote ops
3. If you need a custom selection key, rename it away from 'then'

Example fix

// before
Transforms.setSelection(editor, propsWithThen as any)

// after
delete propsWithThen['then']
Transforms.setSelection(editor, propsWithThen as Partial<Range>)

if ('then' in props && typeof props['then'] === 'function') {
  throw new Error('then must not be a function on selection')
}
const { then, ...rest } = props as any
Transforms.setSelection(editor, rest)

const hasFunctionThen = (o: object): boolean =>
  'then' in o && typeof (o as any).then === 'function'

When it happens

Trigger: Transforms.setSelection(editor, { then: () => {} }) or applying a set_selection op whose properties include then: function; usually from deserialized/untrusted data where 'then' is a function value.

Common situations: Applying remote or localStorage-persisted operations that were not sanitized; JSON payloads where a key named 'then' resolved to a function via eval-like revival; prototype-pollution attempts.

Related errors

• Cannot set the "${key}" property of the selection!
• Cannot resolve a Slate range from a DOM event: ${event}
• Cannot resolve a DOM point from Slate point: ${Scrubber.stri
• Cannot resolve a Slate point from DOM point: ${domPoint}
• Cannot resolve a Slate range from DOM range: ${domRange}

AI-assisted analysis of ianstormtaylor/slate@72a37c701e (2026-08-27). Data as JSON: /api/errors/4fc07a96e9a4f781. Report an issue: GitHub.