findMissingNumber.js

function missingNumber(arr){
  var n = arr.length+1, 
  sum = 0,
  expectedSum = n* (n+1)/2;
  
  sum = arr.reduce((total, num) => total + num);
  return expectedSum - sum;
}

missingNumber([5, 2, 6, 1, 3]);
// 4