javascript find missing number in array

You can do this with the use of indexOf function:

var a = [5],
  count = 5;
var missing = new Array();

for (var i = 1; i <= count; i++) {
  if (a.indexOf(i) == -1) {
    missing.push(i);
  }
}
console.log(missing); // to check the result.

Better way to deal with dynamic MIN & MAX numbers to find range of missing number in an array

const findMissing = num => {
  const max = Math.max(...num); // Will find highest number
  const min = Math.min(...num); // Will find lowest number
  const missing = []

  for(let i=min; i<= max; i++) {
    if(!num.includes(i)) { // Checking whether i(current value) present in num(argument)
      missing.push(i); // Adding numbers which are not in num(argument) array
    }
  }
  return missing;
}

findMissing([1,15]);