Despite the imprecise nature of the question, here's my interpretive answer.
var html = [
'<div> A line</div>',
'<div> Add more lines</div>',
'<div> To the array as you need.</div>'
].join('');
var div = document.createElement('div');
div.setAttribute('class', 'post block bc2');
div.innerHTML = html;
document.getElementById('posts').appendChild(div);