Each column type in Fuse has built-in validations. You refer to the documentation of a specific column type to see the available options.
onValidateRecord hook on your importer.
onValidateRecord(record)
The onValidationRecord hook is called for each row (record) in the importer. It allows you to display front-end validations in the importer for the user to fix.
The record object passed to onValidateRecord contains all of the values for a single row in the importer. You can access values in the record object using the internal_key of each column in your importer.
The onValidateRecord hook returns an errors object with two properties:
errorsblock the user from submitting an import. These are items that must be fixed.warningsprovide a warning message to the user about potential issues. However, a user can submit an import with warnings present.