window.onload = function(){
    blok = document.getElementById('content_text');
    parent = blok.parentNode;
    blok_height = blok.style.height ? blok.style.height : blok.offsetHeight;

    if(blok_height > 60){
        blok.style.maxHeight = '80px';
        link = parent.getElementsByClassName('read-next')[0];
        link.style.display = 'inline';

        link.onclick = function(){

            if(blok.style.maxHeight){
               blok.style.maxHeight = '';
               shadow.style.display ='none';
               link.innerHTML = 'Скрыть';
            } else {
               blok.style.maxHeight = '80px';
               link.innerHTML = 'Читать далее...';
               shadow.style.display ='block';
            }
            return false;
        }
    }
}

[].forEach.call(
    document.querySelectorAll('.block'),
    function (el) {
        ...
    }
);