Return Index of Reduced Array in JavaScript

Question

I have this payload which I'm trying to execute some functions on in JavaScript:

{
  "markets": [
    {
      "mtId": 27,
      "id": 1,
      "cId": "27-1",
      "mt": "OVER_UNDER",
      "selections": [
        {
          "id": 1,
          "price": 1.0896773820446435,
          "selectionStatus": 1,
          "name": "Over 0.5"
        },
        {
          "id": 2,
          "price": 12.159031085167172,
          "selectionStatus": 1,
          "name": "Under 0.5"
        }
      ],
      "marketName": "Over Under 0.5"
    },
    {
      "mtId": 27,
      "id": 2,
      "cId": "27-2",
      "mt": "OVER_UNDER",
      "selections": [
        {
          "id": 1,
          "price": 1.4531444546407393,
          "selectionStatus": 1,
          "name": "Over 1.5"
        },
        {
          "id": 2,
          "price": 3.207355058969988,
          "selectionStatus": 1,
          "name": "Under 1.5"
        }
      ],
      "marketName": "Over Under 1.5"
    },
    {
      "mtId": 27,
      "id": 3,
      "cId": "27-3",
      "mt": "OVER_UNDER",
      "selections": [
        {
          "id": 1,
          "price": 2.3859593325595307,
          "selectionStatus": 1,
          "name": "Over 2.5"
        },
        {
          "id": 2,
          "price": 1.721681322609395,
          "selectionStatus": 1,
          "name": "Under 2.5"
        }
      ],
      "marketName": "Over Under 2.5"
    }
  ]
}

Basically, I want to take the markets array, map the selections and

return the array with price differences from 0.5.
I'd then like to take that array and return the value which is closest to zero.

I can get the result from this function:

export const generateNumberClosestToZero = (markets) => {
  let goal = 0;

  let result = markets
    .map((market) =>
      market.selections.reduce(function (bookPercentage, selection) {
        return 0.5 - 1 / selection.price;
      }, 0)
    )
    .reduce(function (prev, current) {
      return Math.abs(current - goal) < Math.abs(prev - goal) ? current : prev;
    });
  return result;
};

But what I want to do is also retrieve the index of that reduced value, so I can perform operations on that specific market (e.g. markets[1] etc. ). Is there a way to do this with my code, or a better solution than what I have so far?

Nina Scholz · Accepted Answer · 2021-05-11 08:50:18Z

You could map the index along with the value and reduce only by the value.

const
    generateNumberClosestToZero = (markets) => {
        let goal = 0;
        return markets
            .map((market, index) => [
                market.selections.reduce((bookPercentage, selection) => 0.5 - 1 / selection.price, 0),
                index
            ])
            .reduce((a, b) => Math.abs(b[0] - goal) < Math.abs(a[0] - goal) ? b : a);
    },
    data = { markets: [{ mtId: 27, id: 1, cId: "27-1", mt: "OVER_UNDER", selections: [{ id: 1, price: 1.0896773820446435, selectionStatus: 1, name: "Over 0.5" }, { id: 2, price: 12.159031085167172, selectionStatus: 1, name: "Under 0.5" }], marketName: "Over Under 0.5" }, { mtId: 27, id: 2, cId: "27-2", mt: "OVER_UNDER", selections: [{ id: 1, price: 1.4531444546407393, selectionStatus: 1, name: "Over 1.5" }, { id: 2, price: 3.207355058969988, selectionStatus: 1, name: "Under 1.5" }], marketName: "Over Under 1.5" }, { mtId: 27, id: 3, cId: "27-3", mt: "OVER_UNDER", selections: [{ id: 1, price: 2.3859593325595307, selectionStatus: 1, name: "Over 2.5" }, { id: 2, price: 1.721681322609395, selectionStatus: 1, name: "Under 2.5" }], marketName: "Over Under 2.5" }] };

console.log(generateNumberClosestToZero(data.markets));

LaytonGB · Accepted Answer · 2021-05-11 08:55:28Z

I think this is what you're looking for.

const markets = [{"mtId": 27,"id": 1,"cId": "27-1","mt": "OVER_UNDER","selections": [{"id": 1,"price": 1.0896773820446435,"selectionStatus": 1,"name": "Over 0.5"},{"id": 2,"price": 12.159031085167172,"selectionStatus": 1,"name": "Under 0.5"}],"marketName": "Over Under 0.5"},{"mtId": 27,"id": 2,"cId": "27-2","mt": "OVER_UNDER","selections": [{"id": 1,"price": 1.4531444546407393,"selectionStatus": 1,"name": "Over 1.5"},{"id": 2,"price": 3.207355058969988,"selectionStatus": 1,"name": "Under 1.5"}],"marketName": "Over Under 1.5"},{"mtId": 27,"id": 3,"cId": "27-3","mt": "OVER_UNDER","selections": [{"id": 1,"price": 2.3859593325595307,"selectionStatus": 1,"name": "Over 2.5"},{"id": 2,"price": 1.721681322609395,"selectionStatus": 1,"name": "Under 2.5"}],"marketName": "Over Under 2.5"}];

const generateNumberClosestToZero = (markets) => {
  let closest = 9999; // closest value found so far
  let index = []; // output
  markets.forEach((m, n) => {
    m.selections.forEach((s, t) => {
      if (s.price < closest) { // if closer value found
        closest = s.price; // set closest value to new value
        index[0] = n; // store market index
        index[1] = t; // store selections index
      }
    })
  });
  return index;
};

const closest = generateNumberClosestToZero(markets);

console.log(closest); // get indexes
console.log(markets[closest[0]].selections[closest[1]]); // access results from market

Output:

[
  0,
  0
]
{
  "id": 1,
  "price": 1.0896773820446435,
  "selectionStatus": 1,
  "name": "Over 0.5"
}

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