In jquery :

codice:
 jQuery(document).ready(function ($) {
            $(".test1").change(function (e) {
                alert(`test1 = ${e.target.value}`)
            })
            function cambiaTesto(testo) {
                test1.value = testo
                $(".test1").trigger('change')
            }
            cambiaTesto('nuovo testo')
        })
In js vanilla:

codice:
window.addEventListener('DOMContentLoaded', () => {
            const test1 = document.getElementsByClassName('test1')[0]
            test1.addEventListener('change', (e) => {
                console.log(`test1 = ${e.target.value}`)
            })
            function cambiaTesto(testo) {
                test1.value = testo
                test1.dispatchEvent(new Event("change"))
            }
            cambiaTesto('nuovo testo')
        })