JS: Reduce array to nested objects

Question

So I have this array

var mapped = [[2016, "October", "Monday", {object}], [2017, "January", "Friday", {object}], [2017, "January", "Wednesday", {object}], [2017, "October", "Monday", {object}]]

What I want to accomplish is something like this:

[{
    "2016": [{
        "October": [{
            "Monday": [{object}]
        }]
    }],
}, {
    "2017": [{
        "January": [{
            "Friday": [{object}]
        }, {
            "Wednesday": [{object}]
        }]
    }, {
        "October": [{
            "Monday": [{object}]
        }]
    }]
}]

I've been searching around for so long, and I can't find a solution.. By using reduce, I'm getting something like this:

[
    2016: [{
        "month": "October"
    }]
],
[
    2017: [{
        "month": "January"
    },
    {
        "month": "January"
    },
    {
        "month": "September"
    }]
]

So it seems like I'm into something, but still so far away... This is what I'm doing:

mapped.reduce((years, array) => {

                    years[array[0]] = years[array[0]] || [];

                    years[array[0]].push({
                        month: array[1]
                    })

                    return years;

                }, [])

Community · Accepted Answer · 2020-06-20 09:12:55Z

Not exactly what you specified, but I feel the following script outputs a format that will be most useful -- it only produces arrays at the deepest level:

const  mapped = [[2016, "October", "Monday", { a: 1 }], [2017, "January", "Friday", { a: 1 }], [2017, "January", "Wednesday", { a: 1 }], [2017, "October", "Monday", { a: 1 }]];
   
const result = mapped.reduce( (acc, [year, month, day, object]) => {
    let curr = acc[year] = acc[year] || {};
    curr = curr[month] = curr[month] || {};
    curr = curr[day] = curr[day] || [];
    curr.push(object);
    return acc;
}, {});

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

With array wrapping

If you really need the wrapping arrays, you can apply an extra recursive function to the previous result:

const  mapped = [[2016, "October", "Monday", { a: 1 }], [2017, "January", "Friday", { a: 1 }], [2017, "January", "Wednesday", { a: 1 }], [2017, "October", "Monday", { a: 1 }]];
   
const result = mapped.reduce( (acc, [year, month, day, object]) => {
    let curr = acc[year] = acc[year] || {};
    curr = curr[month] = curr[month] || {};
    curr = curr[day] = curr[day] || [];
    curr.push(object);
    return acc;
}, {});

function wrapInArrays(data) {
    return Array.isArray(data) ? data 
        : Object.entries(data).map ( ([key, value]) => {
            return { [key]: wrapInArrays(value) };
        });
}

const wrapped = wrapInArrays(result);
console.log(wrapped);

.as-console-wrapper { max-height: 100% !important; top: 0; }

You sir, are goddamn amazing. I've learned a lot from this answer. Thank you so much.

Nandu Kalidindi · Accepted Answer · 2017-12-15 23:11:48Z

To achieve the same exact structure you can try the strategy below. This will work upto any level of nesting and will continue to build the nested object. Please have a look at the example to see various levels of nesting which will be handled generically.

Map each element into the corresponding nested structure (Map with reduce)
Combine each element from mapped array into a single object to merge all redundant keys
Convert the final nested object into the desired array structure.

const mapped = [[2016, "October", "Monday", "Morning", {time: "10AM"}], [2017, "January", "Friday", {object: "OBJECT"}], [2017, "January", "Wednesday", {object: "OBJECT"}], [2017, "October", "Monday", {object: "OBJECT"}]];

// Map all objects to the respective nested structure
const nestedMap = mapped.map(elem => {
  let reversed = [...elem.reverse()];
  let firstElem = reversed.splice(0, 1)[0];
  return reversed.reduce((acc, elem) => {let newAcc = {}; newAcc[elem] = [acc]; return {...newAcc}}, firstElem);
})

// Combine array to form a single object
const nestedObj = nestedMap.reduce((acc, elem) => {
  let firstKey = Object.keys(elem)[0];
  acc[firstKey]
    ?
    acc[firstKey].push(elem[firstKey][0])
    :
    acc[firstKey] = elem[firstKey];
  return acc;
}, {})

// Convert back into the array with each object as element
const nestedFinalArr = Object.keys(nestedObj).map(key => ({[key]: nestedObj[key]}))

console.log(nestedFinalArr);

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