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\", {obje         


        

Answer 1

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);
   

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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);

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Answer 2

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);