'let' instead of 'const'

const are constants, they can not be changed once declared. let are variables, and can be changed once declared.

I wondered those questions myself. Thanks for the explanation.

Like Zimri states, const can not be changed once declared, while let can. In the case of the sections after "New Article Validation Errors" of the instructions, which I assume you are referring to here:

Before the changes in these sections, we merely declared article as a const, like so:

   const article = await Article.create(req.body);
   res.redirect("/articles/" + article.id);

... and that was fine. Since no further assignments where needed, we could keep article as a const. But, now, we're also opening up the possibility of a second assignment in the updated catch method:

/* POST create article. */
router.post('/', asyncHandler(async (req, res) => {
  let article;
  try {
    article = await Article.create(req.body);
    res.redirect("/articles/" + article.id);
  } catch (error) {
    if(error.name === "SequelizeValidationError") { // checking the error
      article = await Article.build(req.body);
      res.render("articles/new", { article, errors: error.errors, title: "New Article" })
    } else {

    }  
  }
}));

Since article first gets assigned in try, we can't then assign to it in catch when things inevitably go wrong. We'd get an error as we tried to assign to a constant. So, const is out. Unfortunately, we can't just declare a let variable in the try method:

  try {
    let article = await Article.create(req.body);
    res.redirect("/articles/" + article.id);
  }

... because let (and const) are both block-scoped. Meaning it wouldn't carry over to the catch block. Which is why we're now declaring let article above the try-catch block and then assigning as we go through.