interview-1.js

const data = [
    {
        transactionId: "t-100",
        userId: 123,
        category: "appliances",
        total: 22.4,
    },
    {
        transactionId: "t-101",
        userId: 123,
        category: "electronics",
        total: 120.8,
    },
    {
        transactionId: "t-102",
        userId: 122,
        category: "appliances",
        total: 280.4,
    },
    {
        transactionId: "t-103",
        userId: 122,
        category: "electronics",
        total: 0.4,
    },
    {
        transactionId: "t-104",
        userId: 121,
        category: "appliances",
        total: 1800,
    },
    {
        transactionId: "t-105",
        userId: 124,
        category: "electronics",
        total: 1222,
    },
];

class transactionsUtils {

    static highestTransactionId = (arr) => {
        const reducer = (highestObj, currentObj) => highestObj = currentObj.total > highestObj.total ? currentObj : highestObj;
        return arr.reduce(reducer, arr[0]).transactionId;
    }

    static _highestSumOfUnit = (arr, property) => { //helper function
        const sums = {};
        arr.forEach(transaction => {
            sums[transaction[property]] = sums[transaction[property]] == undefined ? transaction.total : sums[transaction[property]] + transaction.total;
        });
        return Object.keys(sums).reduce((highestSum, current) => highestSum = sums[current] > sums[highestSum] ? current : highestSum, Object.keys(sums)[0])
    }

    static highestSumOfTransactionsUser = (arr) => this._highestSumOfUnit(arr, 'userId')

    static highestSalesCategory = (arr) => this._highestSumOfUnit(arr, 'category')
}

console.log(transactionsUtils.highestTransactionId(data))
console.log(transactionsUtils.highestSumOfTransactionsUser(data))
console.log(transactionsUtils.highestSalesCategory(data))