function traverse(element, callback) {
  // assume children are in array element.children...?
  // precondition: if no children, array exists and is empty
  callback(element)
  element.children.forEach(child => traverse(child, callback))
}

function cb(elem) {
  console.log(elem);
};

const myElem = {foo:"bar", children:[
  {foo1:"bar", children:[
    {foo1a:"bar"},
    {foo1b:"bar"}
  ]},
  {foo2:"bar", children:[
    {foo2a:"bar"},
    {foo2b:"bar"}
  ]}
]};

traverse(myElem, cb);

function traverse(element, callback) {
  // assume children are in array element.children...?
  // precondition: if no children, array exists and is empty
  callback(element)
  if (element.children) element.children.forEach(child => traverse(child, callback))
}

Object {
  children: [Object {
  children: [Object {
  foo1a: "bar"
}, Object {
  foo1b: "bar"
}],
  foo1: "bar"
}, Object {
  children: [Object {
  foo2a: "bar"
}, Object {
  foo2b: "bar"
}],
  foo2: "bar"
}],
  foo: "bar"
}

Object {
  children: [Object {
  foo1a: "bar"
}, Object {
  foo1b: "bar"
}],
  foo1: "bar"
}
Object {
  foo1a: "bar"
}
Object {
  foo1b: "bar"
}

Object {
  children: [Object {
  foo2a: "bar"
}, Object {
  foo2b: "bar"
}],
  foo2: "bar"
}
Object {
  foo2a: "bar"
}
Object {
  foo2b: "bar"
}

function traverse(element, callback) {
  callback(element)
  if (element.hasOwnProperty('children') && children.length) {
    for (var i=0; i<children.length; i++) {
      traverse(element.children[i], callback)
    }
  }
}