find missing no. in series

function findMissingNumber(arr) {
let commonDifference = arr[1] - arr[0];
let missingNumber = null;

for (let i = 1; i < arr.length; i++) {
if (arr[i] - arr[i - 1] !== commonDifference) {
missingNumber = arr[i - 1] + commonDifference;
break;
}
}

return missingNumber;
}

// Example usage:
const series1 = [1, 2, 3, 4, 6, 7, 8]; // Missing number: 5
const series2 = [2, 4, 8, 10]; // Missing number: 12
const series3 = [1, 3, 5, 9, 11]; // Missing number: 7

console.log("Missing number in series1:", findMissingNumber(series1));
console.log("Missing number in series2:", findMissingNumber(series2));
console.log("Missing number in series3:", findMissingNumber(series3));