Here is the solution with the js code:

function sumByTown(input) {
let sums = {};
for (let i = 0; i < input.length; i+=2) {
let [town, income] = [input[i], Number(input[i + 1])];
if (sums[town] == undefined) {
sums[town] = income;
} else {
sums[town] += income;
}
}
console.log(JSON.stringify(sums));
}
sumByTown([
"Sofia",
"20",
"Varna",
"3",
"Sofia",
"5",
"Varna",
"4"
]);