JavaScript: constraint satisfaction by backtracking

const regions = ["WA", "NT", "SA", "Q", "NSW", "V", "T"];
const borders = [
  ["WA", "NT"], ["WA", "SA"], ["NT", "SA"], ["NT", "Q"], ["SA", "Q"],
  ["SA", "NSW"], ["SA", "V"], ["Q", "NSW"], ["NSW", "V"],
];
const colors = ["red", "green", "blue"];

function ok(assignment, region, color) {
  return borders.every(([a, b]) => {
    if (a === region && assignment[b] === color) return false;
    if (b === region && assignment[a] === color) return false;
    return true;
  });
}

let count = 0;
let first = null;
function solve(index, assignment) {
  if (index === regions.length) {
    count++;
    if (first === null) first = { ...assignment };
    return;
  }
  const region = regions[index];
  for (const color of colors) {
    if (ok(assignment, region, color)) {
      assignment[region] = color;
      solve(index + 1, assignment);
      delete assignment[region];
    }
  }
}

solve(0, {});
console.log("solutions: " + count);
console.log(regions.map((r) => r + "=" + first[r]).join(" "));