The ?? operator returns the right side only if the left is null or undefined:
null ?? "fallback" // "fallback"
undefined ?? "fallback" // "fallback"
0 ?? "fallback" // 0 (not null/undefined)
"" ?? "fallback" // "" (not null/undefined)
Compare to || which treats all falsy values as missing:
0 || "fallback" // "fallback" (0 is falsy)
0 ?? "fallback" // 0 (0 is not null/undefined)