in src/geojson2h3.js [40:52]
function flatten(arrays) {
let out = null;
for (let i = 0; i < arrays.length; i++) {
if (out !== null) {
for (let j = 0; j < arrays[i].length; j++) {
out.push(arrays[i][j]);
}
} else {
out = arrays[i];
}
}
return Array.from(new Set(out));
}