How to turn array into an nested array of objects and arrays?

Question

I want to combine objects that have the same category number into an array of course s for that category.
But I want the final form to be an array of nested arrays and objects.

I have an array with the following data for example:

var arr = [
    { category : 1, course : "course 1" },
    { category : 1, course : "course 2" },
    { category : 2, course : "course 3" }
]

I want the new array to look like:

output = [
  { category : 1,
    courses : [
      { course : "course 1" },
      { course : "course 2" }
    ]
  },
  { category : 2,
    courses : [
      { course : "course 3" }
    ]
  }
]

Current Code:

var output = [], cnt = 0, i;

for (i = 0; i < arr.length; i++) {
  output[category] = this.coursesData[i];
  cnt++;
}

Current Output:

output = [
    1 : { category : 1, course : "course 2" },
    2 : { category : 2, course : "course 3" },
]

Desired Output:

all the courses with same category need to be merged under that particular category itself.

output = [
  { category : 1,
    courses : [
      { course : "course 1" },
      { course : "course 2" }
    ]
  },
  { category : 2,
    courses : [
      { course : "course 3" }
    ]
  }
]

I have tried merging the array with same id (category in my case) but the desired output is not obtained.

Answer 1

You could try something like this:

const arr = [
    { category: 1, course: "course 1" },
    { category: 1, course: "course 2" },
    { category: 2, course: "course 3" }
];

function transform(arr) {
    const output = {};

    for (let i = 0; i < arr.length; i++) {
        let course = arr[i];
        if (output[course.category]) {
            output[course.category].push(course);
        } else {
            output[course.category] = [];
            output[course.category].push(course);
        }
    }

    const result = [];
    for (let key in output) {
        result.push({
            "category": key,
            "courseList": output[key].map(o => { return { "course": o.course }; })
        })
    }
    console.log(result);
}

transform(arr);

Using Lodash or similar libraries, the logic can be well optimised but for basic understanding, have written code in basic JS