let data = [
  {
    title: "menu 1",
    children :[
      { title: "menu 1.1"},
      {
        title: "menu 1.2",
        children: [
          {title: "menu 1.2.1"},
          {title: "menu 1.2.2"},
        ]
      },
    ]
  },
  {
    title: "menu 2",
    children :[
      { title: "menu 2.1"},
      { title: "menu 2.2"},
    ]
  }
]

function buildTree(data, isChild = false) {
  let html = '<ul>'
  data.forEach(element => {
    html += `<li>${d.title}</li>`
    // If the current data element
    // has children then call the
    // buildTree again passing in
    // the children and isChild = true
    if(d.children) {
      html += buildTree(d.children, true)
    }
  });
  html += '</ul>'
  return html
}

let uls = buildTree(data);
console.log(uls);