Discussion on: If/else or just if?

As long as every branch returns then the 2nd version is fine with me.
(Although I'd still use brackets instead of guard clauses because of personal preference regarding readability)

It becomes a problem when the if does not return and gets more complex than a simple one liner.
I.e.

if (env.DEBUG) {
    Logger.log(/* message */);
}
// Rest of logic here

would obviously be okay, but

let var1;
let var2;
let var3;

if (/* Some condition */) {
    // Many lines
    // of code
    // that affect 
    // the state of 
    // the variables
}

// Rest of logic
// working on the 
// variables unconditionally,
// here

return result;

would not be okay, because it makes it really hard to reason about the output of the function and the logical flow of your data can't be parsed at a glance.