Find missing element in array

const k = [1,2,3,5,7,9,0];
let missingElement;

for (let i = 1; i < k.length ; i++) {
const a = k[i] - k[i - 1];
const b = k[i + 1] - k[i];

if (a !== b) {
let c;a

if (a > b) {
  c = a - b;
} else {
  c = b - a;
}

missingElement = k[i] + c;
break;

}
}

console.log("Missing element:", missingElement);