I think the post doesn't give a fair comparison as in the if/else case there's no need to use "else" blocks if you're throwing an error or returning. In this case I think simple "if" statements are cleaner and certainly more "normal".

E.g.

  if (!user) {
    throw new Error("User must be defined.");
  } 

  if (!user.firstName) {
    throw new Error("User's first name must be defined");
  }

  return user;

When validation gets complex (e.g. there are many criteria to check), I like to build a list/stream/array (what ever the language offers) of tuples of predicates (functions from the object that gets validated to boolean) and strings (or functions from the object to string so I can have context in my error messages).

Then iterate over the tuples, if a predicate fails, return the associated error message and throw an error/display the message to the user.

In the end it looks something like this:

  var validators = Stream.of(
    Map.entry(user -> user != null, "User must be defined"),
    Map.entry(user -> user.firstName != null, "Missing first name"))

  validators.filter(e -> e.getKey().apply(userToBeValidated)).map(Map.Entry::getValue).getFirst()

This limits branching and you have all validation criteria neatly organized in the same location.

That approach looks nice though. On that subject, JS has some nice libraries including io-ts[1] which has a functional approach using Eithers to encapsulate errors/success.

[1]: https://github.com/gcanti/io-ts