var categories = [
  {id:1, parent:null},
  {id:2, parent:1},
  {id:3, parent:null},
  {id:4, parent:2},
  {id:5, parent:2},
  {id:6, parent:3},
  {id:7, parent:null},
  {id:8, parent:7},
  {id:9, parent:5},
];

var categories_map = {}
categories.forEach(function (category) {
  categories_map[category.id] = category;
});

console.log(categories_map);

categories.forEach(function (category) {
  if (category.parent) {
    categories_map[category.parent]['subcategories'] = categories_map[category.parent]['subcategories'] || [];
    categories_map[category.parent]['subcategories'].push(category);
  }
});

console.log(categories);

categories = categories.filter(function (category) {
  return !category.parent;
});

console.log(JSON.stringify(categories, null, '    '));