let posts = {
    'first post!': 'first-post',
}

Object.keys(posts).forEach(post => {
    if (post.length > 35)
        document.getElementById('posts_list').innerHTML += `
<li><a href="/site/blog.html?post=${posts[post]}">${post.slice(0, 35)}...</a></li>
    `
    else
        document.getElementById('posts_list').innerHTML += `
<li><a href="/site/blog.html?post=${posts[post]}">${post}</a></li>
    `
});