Fix the in-order visiting too early — find the bug

const tree = { v: 2, left: { v: 1, left: null, right: null }, right: { v: 3, left: null, right: null } };
const out = [];
function walk(n) { if (!n) return; out.push(n.v); walk(n.left); walk(n.right); }
walk(tree);
document.querySelector('#out').textContent = out.join("-");