It appears that the only way to get a nullish coalescing operator (??) is to enable the --nullish option, but this is a global option, so every use of a logical or or || turns into a ??. This means that normal logical tests are broken.

if (a == 1) || (a == 2) && (b== 3)
  ...stuff
end

results in

if ((a === 1) ?? (a === 2) && (b === 3)) {
  ...stuff
}

which will throw a syntax error as it's nonsense.

The problem is that there doesn't seem to be a way of using nullish coalescing without this option.