Is the !-- notation represents a special operator

No,that's not a special operator. But it is a combination of 2 standard operators one after the other,

1. A logical not (!)
2. A prefix decrement (--)

At first, the value decremented by one and then tested to see if it is equal to zero or not for determining the truthy/falsy value.

Practical Code Example

// '<!--' is NOT a special operator. It is legacy HTML comment syntax:
// In browser script tags, it is parsed identically to a single line '//' comment:
<!-- console.log('This line is commented out in browsers');
console.log('This line executes');