I grabbed the code from the article and annotated it with the different cases from the famous picture*

  switch (expr.kind) {
    case "number"/"string"/"var":
      ... [Var]
    case "call":
      ... [App]
    case "function":
      throw new Error("...[Abs]")
    case "let":
      ... [Let]

Looks like most of the hard work's done, and probably wouldn't be too tricky to get [Abs] in there too!

* https://wikimedia.org/api/rest_v1/media/math/render/svg/10d6...