what is the ?? two question marks in javascript code example

Example: javascript double question mark

let a = null;
const b = a ?? -1;		// Same as b = ( a != null ? a : -1 );
console.log(b);		  	// output: -1
//OR IF
let a = 9;
const b = a ?? -1;
console.log(b);  		// output: 9

//PS.,VERY CLOSE TO '||' OPERATION IN FUNCTION, BY NOT THE SAME