let arra=[1,2,3,4,6]// 4 is missing, given n-1 aaray[i.e 3], range till 4 function findMissingNum(arr){ let n=arr.length+1; let totalSum=0; let actualArrSum=n*(n+1)/2; for(let a=0;a<arr.length;a++){ totalSum=arr[a]+totalSum; } return actualArrSum-totalSum; } console.log('Missing Num',findMissingNum(arra));