Looping nested arrays get the sum in JavaScript

Question

I am working on a Javascript array where I need to loop the 2nd array in the main array and get the total sum of a value of a particular field. below is what my JSON looks like. here I need to loop Array and get the sum of ps_lessons.lesson_length_minutes I need the O/P as 118 (20 + 22 + 33 + 43)

[
    {
        "sort_id": 1,
        "category": "Introduction",
        "ps_lessons": [
            {
                "id": "48ca672b-aaca-473c-b54b-3cf699da848b",
                "sort_id": 1,
                "lesson_title": "Welcome to the course",
                "lesson_length_minutes": 20
            },
            {
                "id": "3f0f5715-a442-4262-ad32-6fb653df62c2",
                "sort_id": 2,
                "lesson_title": "What is React JS?",
                "lesson_length_minutes": 22
            }
        ]
    },
    {
        "sort_id": 2,
        "category": "Fundamentals",
        "ps_lessons": [
            {
                "id": "e28abd67-29a4-4bc9-a415-fb5db0aabfa9",
                "sort_id": 4,
                "lesson_title": "Fundamentals of React JS",
                "lesson_length_minutes": 33
            },
            {
                "id": "1d9668da-94e6-4085-bd89-176cc4e8e62d",
                "sort_id": 3,
                "lesson_title": "Fundamentals of JSX",
                "lesson_length_minutes": 43
            }
        ]
    }
]

I tried something like the below by using the array method map 2 times but it's not working. can anyone suggest how to do this?

const extract_mins_and_sum = (result) => {
  console.log('result', result);
     result.map((single_row) =>
      let sum = 0;
        single_row.ps_lessons.map((single_row_lesson) =>
    
         ( console.log(single_row_lesson.lesson_length_minutes);
          sum += single_row_lesson.lesson_length_minutes)
        )
      );
    };

Thanks Venk

NightEye · Accepted Answer · 2022-06-06 12:55:08Z

1

You can try map, flat and reduce.

const extract_mins_and_sum = (result) => {
  return result.map(x => x.ps_lessons.map(y => y.lesson_length_minutes)).flat().reduce((a, b) => a + b, 0)
};

answered Jun 6, 2022 at 12:55

NightEye

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Comments

Catalin-Ioan Narita · Accepted Answer · 2022-06-06 12:52:42Z

1

The straightforward way is to loop through each element of the big array and get the sum by using a reduce function on each ps_lessons array.

const extract_mins_and_sum = (result) => {
    let sum = 0;
    result.forEach((elem) => {
        sum += elem.ps_lessons.reduce((acc, val) => acc + val.lesson_length_minutes, 0);
    });

    return sum;
}

Map is generally used to map an array to another array, but preserving metadata like array length, order etc. Reduce is generally used to, well, reduce an array to a single value, based on whatever logic you use in the callback function. On this example, reduce is used to transform an array into a sum of some numeric fields.

answered Jun 6, 2022 at 12:52

Catalin-Ioan Narita

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Comments

Rahul Sharma · Accepted Answer · 2022-06-06 12:58:34Z

1

You could use a solution using map and reduce extracting the value to an external variable like so:

const data = [
  {
    sort_id: 1,
    category: "Introduction",
    ps_lessons: [
      {
        id: "48ca672b-aaca-473c-b54b-3cf699da848b",
        sort_id: 1,
        lesson_title: "Welcome to the course",
        lesson_length_minutes: 20,
      },
      {
        id: "3f0f5715-a442-4262-ad32-6fb653df62c2",
        sort_id: 2,
        lesson_title: "What is React JS?",
        lesson_length_minutes: 22,
      },
    ],
  },
  {
    sort_id: 2,
    category: "Fundamentals",
    ps_lessons: [
      {
        id: "e28abd67-29a4-4bc9-a415-fb5db0aabfa9",
        sort_id: 4,
        lesson_title: "Fundamentals of React JS",
        lesson_length_minutes: 33,
      },
      {
        id: "1d9668da-94e6-4085-bd89-176cc4e8e62d",
        sort_id: 3,
        lesson_title: "Fundamentals of JSX",
        lesson_length_minutes: 43,
      },
    ],
  },
];

const sumMinutes = (lessons) => {
  return lessons.reduce((acc, ps) => {
    return (acc + (ps.ps_lessons ? sumMinutes(ps.ps_lessons): ps.lesson_length_minutes));
  }, 0);
};

console.log(sumMinutes(data))

edited Jun 6, 2022 at 12:58

Rahul Sharma

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answered Jun 6, 2022 at 12:53

Jorge Guerreiro

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1 Comment

Rahul Sharma Over a year ago

I'm sorry by mistake I've updated your answer. Don't know how to revert.

Rahul Sharma · Accepted Answer · 2022-06-06 12:59:27Z

You can use reduce function to solve this

const data = [
  {
    sort_id: 1,
    category: "Introduction",
    ps_lessons: [
      {
        id: "48ca672b-aaca-473c-b54b-3cf699da848b",
        sort_id: 1,
        lesson_title: "Welcome to the course",
        lesson_length_minutes: 20,
      },
      {
        id: "3f0f5715-a442-4262-ad32-6fb653df62c2",
        sort_id: 2,
        lesson_title: "What is React JS?",
        lesson_length_minutes: 22,
      },
    ],
  },
  {
    sort_id: 2,
    category: "Fundamentals",
    ps_lessons: [
      {
        id: "e28abd67-29a4-4bc9-a415-fb5db0aabfa9",
        sort_id: 4,
        lesson_title: "Fundamentals of React JS",
        lesson_length_minutes: 33,
      },
      {
        id: "1d9668da-94e6-4085-bd89-176cc4e8e62d",
        sort_id: 3,
        lesson_title: "Fundamentals of JSX",
        lesson_length_minutes: 43,
      },
    ],
  },
];

const sumMinutes = (lessons) => {
  return lessons.reduce((acc, ps) => {
    return (acc + (ps.ps_lessons ? sumMinutes(ps.ps_lessons): ps.lesson_length_minutes));
  }, 0);
};

console.log(sumMinutes(data))

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