const data = [{id: 3, p_id: 2}, {id: 4, p_id: 1}, {id: 1, p_id: null}, {id: 5, p_id: 2}, {id: 2, p_id: 1}, {id: 6, p_id: 2}];

const treeData = [];

function buildTree(data, treeData) {
  const findInChildren = (parent, obj) => {
    if (parent.children) {
      const childParentIndex = parent.children.findIndex((o) => o.id === obj.p_id);
      if (childParentIndex > -1) {
        if (!parent.children[childParentIndex].children) {
          parent.children[childParentIndex].children = [];
        }
        parent.children[childParentIndex].children.push(obj);
      } else {
        for (let ci=0; ci<parent.children.length; ci++) {
          return findInChildren(parent.children[ci], obj);
        }
      }
    } else {
      data.push(obj);
    }
    return;
  }
	for (let i=0; i<data.length; i++) {
    if (!data[i].p_id) {
      treeData.push(data[i]);
    } else {
      const parentIndex = treeData.findIndex((o) => o.id === data[i].p_id);
      if (parentIndex > -1) {
        if (!treeData[parentIndex].children) {
          treeData[parentIndex].children = [];
        }
        treeData[parentIndex].children.push(data[i]);
      } else {
        if (!treeData.length) {
          data.push(data[i]);
        } else {
          for (let ti=0; ti<treeData.length; ti++) {
            findInChildren(treeData[ti], data[i]);
          }
        }
      }
    }
  }
  return treeData;
}

const result = buildTree(data, treeData);

console.log(result);