JSFiddle - React, Tailwind, and code Playground
JavaScript
function func(array,val){
return array.sort((a,b) => b - a)
.find((a,i,x) => (x[i] !== x[i-1]-1) && (x[i] <= val || i === x.length-1));
console.log(val);
}
console.log(func([3,5,6],6));
console.log(func([4,5,6],6));