Calculate the total value of a certain field within an array when a specific condition is satisfied

I have two sets of data. I am looking to calculate the total time_spent for each course_id that appears in both set 1 and set 2.

let set1 = [
  { instructor_id: 7, course_id: 19, lesson_id: 1, time_spent: 0 },
  { instructor_id: 7, course_id: 19, lesson_id: 2, time_spent: 0 },
  { instructor_id: 7, course_id: 19, lesson_id: 3, time_spent: 0 },
  { instructor_id: 7, course_id: 20, lesson_id: 4, time_spent: 80 },
  { instructor_id: 7, course_id: 20, lesson_id: 5, time_spent: 40 },
  { instructor_id: 8, course_id: 21, lesson_id: 6, time_spent: 0 },
];

let set2 = [
  { id: 19, title: "Course 19", duration: 180 },
  { id: 20, title: "Course 20", duration: 120 },
];

// expected output
newSet = [
  { instructor_id: 7, course_id: 19, time_spent: 0 },
  { instructor_id: 7, course_id: 20, time_spent: 120 },
];
javascriptarrays

Answer №1

There are numerous methods to achieve this task, and even though the specific actions for all the fields were not clearly defined, utilizing Array.reduce can efficiently carry out a selective sum.

One smart approach is to multiply a value by a boolean in order to sum only the matching values, while adding 0 for non-matching items. The expression condition * value is essentially equivalent to the ternary operation condition ? value : 0

const arr1 = [
  { instructor_id: 7, course_id: 19, lesson_id: 1, time_spent: 0 },
  { instructor_id: 7, course_id: 19, lesson_id: 2, time_spent: 0 },
  { instructor_id: 7, course_id: 19, lesson_id: 3, time_spent: 0 },
  { instructor_id: 7, course_id: 20, lesson_id: 4, time_spent: 80 },
  { instructor_id: 7, course_id: 20, lesson_id: 5, time_spent: 40 },
  { instructor_id: 8, course_id: 21, lesson_id: 6, time_spent: 0 },
];

const arr2 = [
  { id: 19, title: "Course 19", duration: 180 },
  { id: 20, title: "Course 20", duration: 120 },
];

const result = arr2.map(({ id }) => ({
  course_id: id,
  time_spent: arr1.reduce(
    (prev, cur, index) => prev + (cur.course_id == id) * cur.time_spent
    , 0)
}));

console.log(result);

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