bfs.js

var o = {
  value: 1,
  left: {
    value: 2,
    left: {
      value: 4
    },
    right: {
      value: 5
    }
  },
  right: {
    value: 3,
    left: {
      value: 6
    },
    right: {
      value: 7
    }
  }
};

var fn = function (node) {
  var arr = [];

  var l = node.left;
  var r = node.right;

  l && arr.push(l.value);
  r && arr.push(r.value);

  if (l) {
    arr = arr.concat(fn(l));
  }

  if (r) {
    arr = arr.concat(fn(r));
  }

  if (arr.length === 6) {
    arr.unshift(node.value);
  }

  return arr;
};

console.log(fn(o));