Flatten tree into list

const tree = [1, 2, 3, [4, 5, 6], [12, 8], [13, [14, 15]]];
let list = [];

function treeToList(tree) {
     for (const element of tree) {
          if (Array.isArray(element)) {
               treeToList(element);
          } else {
               list.push(element);
          }
     }
}


treeToList(tree) 
console.log("Flat list",list)