From a given array of numbers find the lonely integer

gistfile1.js

function lonelyinteger(a) {
    // Write your code here
    let isUnique = (value, index, self) => self.indexOf(value) === index;
    let distinct = a.filter(isUnique);
    let lonelyint = null;
    distinct.forEach(v => {
        if (a.filter(index => index == v).length == 1) {
            lonelyint = v;
        }
    });
    return lonelyint;
  }

//usage
let arr = [1, 2, 3, 4, 5, 6, 72, 1, 6, 5, 5, 4, 3];
console.log(lonelyinteger(arr));