Restructure single JSON into nested array [duplicate]

Here is a working example of a solution:

const source =  [
    {Id: "1", Id2: "1", Year: "2019", Month: "1", Value: "123"},
    {Id: "1", Id2: "1", Year: "2019", Month: "2", Value: "3"},
    {Id: "1", Id2: "1", Year: "2019", Month: "3", Value: "-123"},
    {Id: "1", Id2: "1", Year: "2019", Month: "4", Value: "-4123"},
    {Id: "1", Id2: "1", Year: "2020", Month: "1", Value: "214"},
    {Id: "1", Id2: "1", Year: "2020", Month: "2", Value: "-41123"},
    {Id: "1", Id2: "1", Year: "2020", Month: "3", Value: "12344"},
    {Id: "1", Id2: "1", Year: "2020", Month: "4", Value: "1232"}
];

const destination = []
const classifiedTokens = [] // will be used to stored already classified tokens

const getNextTokenArray = function(source) {
    let unusedToken = null;
    const nextTokenArray = source.filter(function(element) {
        if (!unusedToken && !classifiedTokens.includes(element["Year"])) {
            unusedToken = element["Year"]
            classifiedTokens.push(unusedToken)
        }
        return unusedToken ? unusedToken === element["Year"] : false;
    });
    return unusedToken ? nextTokenArray : null;
}

const addToDestination = function(tokenArray) {
    if (tokenArray.length === 0) return
    let res = {
        Year : tokenArray[0]["Year"],
        data : []
    }
    tokenArray.forEach((element) => {
        res.data.push({
            Month: element.Month,
            Value: element.Value
        })
    })
    destination.push(res);
}

let nextTokenArray = getNextTokenArray(source);
while (nextTokenArray) {
    addToDestination(nextTokenArray)
    nextTokenArray = getNextTokenArray(source)
}

document.body.appendChild(document.createElement('pre')).innerHTML = JSON.stringify(destination, null, 2);

you can try it directly in the following fidle: https://jsfiddle.net/zgj0fcq9/1/

You can do this with reduce. Reduce is often used to take an array and turn it into a single value, e.g. summing items in an array:

let vs = [1,2,3,4];
console.log(vs.reduce(function(accumulator, value){ return accumulator + value; }, 0));

The second parameter in reduce is the default (or starting) value (in the above that's 0).

So to reduce your array of objects into your array of nested objects, you'll need to check your accumulator to see if the year already exists in the outer array:

let year = accumulator.find(item => item.Year === value.Year);

If year is undefined then you'll need to create a new object with the current year, and give it a data property that's an array with one item that is an object with the Month and Value properties set.

let newYear = { Year: value.Year, data: [{ Month: value.Month, Value: value.Value}]}

Then add that to the accumulator (e.g. with push).

If the year does exist in the accumulator then you'll need to add a new object to the data property:

year.data.push({Month: value.Month, Value: value.Value});

Then you're done.