For given number sum all its digits if sum is not single digit then again sum its digit and retun final single digit.

function createCheckDigit(membershipId) {

// Write the code that goes here.

if (membershipId.toString().length == 1) {

return membershipId

} else {

var t = sumMe(membershipId)

while (t > 9) {

t = sumMe(t)

}

return t;

}

}

function sumMe(membershipId) {

var x = membershipId.toString()

var sum = 0

for (let o = 0; o < x.length; o++) {

sum = sum + parseInt(x[o])

}

return sum;

}

//sample call

console.log(createCheckDigit("777"));

OR

function digiSum(num) {

if (num > 0 && num < 9) {

return num;

} else {

var sum = 0

while (num != 0) {

var lastDigit = num % 10

sum = sum + lastDigit

num = parseInt(num / 10)

}

if(sum > 9){

num = sum;

return digiSum(num)

}

return sum

}

}

var dsum = digiSum(56)

console.log(dsum)

OR

function sumMe(membershipId) {

var x = membershipId.toString()

var sum = 0

for (let o = 0; o < x.length; o++) {

sum = sum + parseInt(x[o])

}

if (sum > 9) {

return sumMe(sum)

} else {

return sum;

}

}

console.log(sumMe("7779999"));