jet-engine-trim-callback.php

<?php

add_filter( 'jet-engine/listings/allowed-callbacks', 'jet_add_trim_callback', 10, 2 );

function jet_add_trim_callback( $callbacks ) {
	$callbacks['jet_trim_string'] = 'Trim string';
	return $callbacks;
}

function jet_trim_string( $string ) {
	return mb_strimwidth( $string, 0, 20, '...' );
}