"??" is called Nullish coalescing operator.
return the right hand side of operator if left hand side is null or undefined.
For example.
const foo = null ?? 'default string';
console.log(foo);
// expected output: "default string" because left hand side is null
const baz = 0 ?? 42;
console.log(baz);
// expected output: 0 . because 0 is not null or undefined.