Add custom fields to User Mutations in WPGraphQL

add-custom-fields-to-user-mutations.php

add_action( 'graphql_register_types', function() {

	$args = [
		'type' => 'String',
		'description' => __( 'Custom field for user mutations', 'your-textdomain' ),
		'resolve' => function( \WP_User $user ) {
		  return get_user_meta( $user->ID, 'custom_field_meta_key', true );
		}
	];

	register_graphql_field( 'User', 'customField', $args );
	register_graphql_field( 'CreateUserInput', 'customField', $args );
	register_graphql_field( 'UpdateUserInput', 'customField', $args );

} );

add_action( 'graphql_user_object_mutation_update_additional_data', function( $user_id, $input, $mutation_name, $context, $info ) {

	if ( ! empty( $input['customField'] ) ) {
		// Consider other sanitization if necessary and validation such as which
		// user role/capability should be able to insert this value, etc.
		$value = sanitize_text_field( $input['customField'] );
		update_user_meta( $user_id, 'custom_field_meta_key', $value );
	}

}, 10, 5 );