findUnit.js

const district = 'barisal';
const customerCategory = 'residential';
const averageElectricityConsumptionCost = 1800; // tk

const unitInfo = {
   barisal: {
      residential: [
         {
            min: 0,
            max: 50,
            perUnitCost: 3.15,
            maxCost: 157.5,
         },
         {
            min: 51,
            max: 100,
            perUnitCost: 3.95,
            maxCost: 395,
         },
         {
            min: 101,
            max: 150,
            perUnitCost: 5.0,
            maxCost: 750,
         },
         {
            min: 151,
            max: 200,
            perUnitCost: 6.8,
            maxCost: 1360,
         },
         {
            min: 201,
            max: 250,
            perUnitCost: 8.0,
            maxCost: 2000,
         },
         {
            min: 0,
            max: 300,
            perUnitCost: 6.2,
         },
      ],
      commercial: [],
      industrial: [],
   },
   khulna: {},
   dhaka: {},
};

const findUnit = (dist, category, cost) => {
   const smallerUnits = unitInfo[dist][category].filter(
      (item, i) => item.maxCost < cost
   );

   const unit = unitInfo[dist][category][smallerUnits.length];
   return unit;
};

console.log(
   findUnit(district, customerCategory, averageElectricityConsumptionCost)
);