How to get accumulator in flatMap array methode in JavaScript(flatmap vs reduce)?

Question

There is an initial array that needs to be mapped and filtered at the same time:

let data = [
        {
            "id": 1,
            "parentId": null,
            "dxId": "1_id",
            "dxParentId": null,
            "defName": "group1"
        },
        {
            "id": 12,
            "parentId": null,
            "dxId": "12_id",
            "dxParentId": null,
            "defName": "группа3"
        },
        {
            "id": 8,
            "parentId": 1,
            "dxId": "1_id/13",
            "dxParentId": "1_id",
            "defName": "group4"
        },
        {
            "id": 5,
            "parentId": 1,
            "dxId": "1_id/20",
            "dxParentId": "1_id",
            "defName": "user1"
        },
        {
            "id": 5,
            "parentId": 1,
            "dxId": "1_id/20",
            "dxParentId": "12_id",
            "defName": "user1"
        },
    ];

I filter by the presence of the parentide property, and collect the resulting array of strings (not the initial array of objects).

originally I did it through reduce methode:

resultArr = data.reduce((filtered, obj) => {
                                if(  obj.dx_parentId !== null) {
                                    if(!(filtered.includes(obj.dxParentId)))  {
                                        filtered.push(obj.dxParentId);
                                    }
                                }
                                return filtered;
                            }, []);
 console.log(resultArr , 'result'); // ['1_id', '12_id'] 'result'

then I discovered for myself that this can be done with the flatMap array method

resultArr =  data.flatMap((obj) =>  obj.dxParentId !== null ? [obj.dxParentId] : []);
console.log(resultArr , 'result'); //['1_id', '1_id', '12_id'] 'result'

If you notice that in the case of reduce I use filtered (accumulating array) I do an additional check using includes and I get the result that I need.

The question is how to do the same it via flatMap? is it possible to access the accumulating result at each iteration?

Additionally: can someone tell me what is better to use to solve this case, in terms of optimization? mb forEach? thanks in advance

Answer 1

You cannot access the array that is being built in flatMap. In mapping, and also mapping-with-flattening, the callback is supposed to return a value that depends only on the particular item.

You might simple not want to use reduce at all for an imperative algorithm like this:

const resultArr = [];
for (const obj of data) {
    if (obj.dxParentId !== null) {
        if (!resultArr.includes(obj.dxParentId)) {
            filtered.push(obj.dxParentId);
        }
    }
}
console.log(resultArr, 'result');

However, to get unique values from an array there is a much better (simpler and more efficient) way:

const resultArr = Array.from(new Set(data.map(obj => obj.dxParentId).filter(id => id !== null)));
console.log(resultArr, 'result');