// nullish coalescing operator
let substitute = 51;

let val1 = null ?? 10;
console.log(val1);  //10

let val2 = undefined ?? substitute;
console.log(val2);  //51

let val3 = 5 ?? 8;
console.log(val3);  //5

let original = null;
let test = undefined;
let val4 = original ?? test ?? substitute
console.log(val4);  //51