findMissinNumber.js

var findMissingNumber = function(arr) {
	let size = arr.length;
    console.log(size);
	if(!size) {
		return false;
	}
  // could sort the array first and get
  // first and last index
  // not sure which one would be faster
	//let lower = Math.min.apply(null, arr);
	let upper = Math.max.apply(null,arr);
   
  // Using Gauss Sum: S = ( upper * (upper + 1) ) / 2;
	let expectedSum = ( upper * (upper + 1) ) / 2;
	//let lowerSum = (lower * (lower - 1)) / 2;
	console.log(expectedSum);
	let currentSum = arr.reduce( (a,c) => a + c, 0 );
	console.log(currentSum);

	let missingNumber = expectedSum - currentSum;

	return missingNumber;
};