let coll = document.getElementsByClassName('collapsible');
for (let i = 0; i < coll.length; i++) {
coll[i].addEventListener('click', function(but) {
this.classList.toggle('active');
let content = this.nextElementSibling;
if (content.style.maxHeight) {
content.style.maxHeight = null;
} else {
content.style.maxHeight = content.scrollHeight + 'px'
}
})
}
<button class="collapsible"> Open</button>