let x = 4;
let y = 16;
let arr = [5, 5, 5, 5, 6, 7, 9, 10, 10, 10, 11, 13];
let missingValues = new Set([...Array(y - x + 1).keys()].map(i => i + x)
.filter(i => !arr.includes(i)));
console.log("missingValues:", missingValues);
console.log("missingValues:", Array.from(missingValues));
/*
run:
missingValues: Set(6) { 4, 8, 12, 14, 15, 16 }
missingValues: [ 4, 8, 12, 14, 15, 16 ]
*/