[WinliteUA]

вот код JS

function showText(el) {
debugger;
if (el.previousElementSiblings.clientHeight === 80) {
el.previousElementSiblings.style.height = "100%";
el.innerHTML = "ShowLess...";
} else {
el.previousElementSiblings.style.height = "80px";
el.innerHTML = "Read More....";
}
}

вот html
<button onClick="showText(this)">Read more...</button>