https://github.com/josevalim/nested-data-structure-traversal

nested-data-structure-traversal.js

const list = [
  {
    "title": "Getting started",
    "reset_lesson_position": false,
    "lessons": [
      {"name": "Welcome"},
      {"name": "Installation"}
    ]
  },

  {
    "title": "Basic operator",
    "reset_lesson_position": false,
    "lessons": [
      {"name": "Addition / Subtraction"},
      {"name": "Multiplication / Division"}
    ]
  },

  {
    "title": "Advanced topics",
    "reset_lesson_position": true,
    "lessons": [
      {"name": "Mutability"},
      {"name": "Immutability"}
    ]
  }
]

list.reduce(
  (agg, curr, index) => {
    let position = curr.reset_lesson_position ? 1 : agg.index;

    return {
      list: [
        ...agg.list,
        {
          ...curr,
          position: index + 1,
          lessons: curr.lessons.map((lesson) => {
            return { ...lesson, position: position++ };
          }),
        },
      ],
      index: position,
    };
  },
  { list: [], index: 1 }
)