Missing Number In Array

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));