Unable to push string into array

First, you declare var posts = new Array(). So posts is an array. Next line (in execution order) : posts = posts.join(). So now posts is an empty string. You are changing the type of the variable, which is a bad practice (Typescript wouldn't let you do that). Now next line in execution order : .then(keys =>. You start pushing stuff into posts, but posts is now a string, remember? Not an array anymore.

You use the async keyword for no reason, since there is no await in it. You might as well leverage it :

exports.load = async function(){
  db.set("hello", {
    "author": "Some author 1",
    "title": "Blog Post 1",
    "content": "First post content",
    "date_posted": "Dec 17, 2021"
  })

  let postsArray = new Array();

  const keys = await db.list();

  keys.forEach(key => {
    postsArray.push(`<article class="media content-section">
      <div class="media-body">
        <div class="article-metadata">
          <a class="mr-2" href="/p">Anonymous</a>
          <small class="text-muted">${db.get(key).date_posted}</small>
        </div>
        <h2><a class="article-title" href="#">${ db.get(key).title }</a></h2>
        <p class="article-content">${ db.get(key).content }</p>
      </div>
    </article`
    )
  })
  
  const posts = postsArray.join()

  fs.writeFileSync("public/posts.ejs", posts)
}

OR with .map() in one line :

exports.load = async function(){
  db.set("hello", {
    "author": "Some author 1",
    "title": "Blog Post 1",
    "content": "First post content",
    "date_posted": "Dec 17, 2021"
  })

  const keys = await db.list();

  const posts = keys.map( key => `<article class="media content-section">....</article`).join();

  fs.writeFileSync("public/posts.ejs", posts)
}