In javascript, null coalescing (??) operator is used to get first "defined" variable from list.

For eg: x=a ?? b

Which is same as saying,
x = (a!=null || a!=undefined)?a:b

Advantage

It reduces amount of code needed to check for null or undefined value.

Limitation

We cannot use ?? Operator along with && or || operator without explicit parenthesis.

Eg: x = a && b ?? c will raise syntax error.

x=(a && b) ?? c
Is correct way

Comparison

We can use (||) operator as replacement to (??). But there is small difference between them.
?? returns first defined value while || returns first truth value.

Reference

https://javascript.info/nullish-coalescing-operator