flatten it into a list

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

for (let i = 0; i < tree.length; i++) {
if (tree[i].length > 0) {
for (let j = 0; j < tree[i].length; j++) {
if (tree[i][j].length > 0) {
for (let k = 0; k < tree[i][j].length; k++) {
treeList.push(tree[i][j][k]);
// console.log('kkkk---------', tree[i][j][k]);
}
} else {
treeList.push(tree[i][j]);
// console.log('jjjjj--------', tree[i][j]);
}
}
} else {
treeList.push(tree[i]);
// console.log('iiiii-------', tree[i]);
}
}
console.log(treeList && treeList);