Dev.to Daily Challenge #4 (July 1, 2019) (See: https://dev.to/thepracticaldev/daily-challenge-4-checkbook-balancing-hei)

challenge.js

const sanitize = str => str.replace(/[^0-9A-Za-z\.\s]/g, "");

const notBlank = str => str !== "";

const extract = str => {
    const [ line, category, expense ] = str.split(/\s+/);
    return { line: Number(line), category, expense: Number(expense) };
};

const byLineAndCategory = (a, b) => a.line < b.line 
    ? -1 : a.line > b.line 
        ? 1 : a.category < b.category 
            ? -1 : a.category > b.category 
                ? 1 : 0;

const balanceReducer = (
    {openingBalance, totalExpenses, entries}, 
    {line, category, expense}
) => {
    const newTotal = totalExpenses + expense;
    const newBalance = openingBalance - newTotal;
    return {
        openingBalance,
        totalExpenses: newTotal,
        averageExpense: newTotal / (entries.length + 1),
        entries: [ ...entries, {line, category, expense, balance: newBalance }]
    }
};

const round2 = n => (Math.round(n * 100) / 100)
    .toLocaleString(undefined, {
        style: "decimal",
        minimumFractionDigits: 2,
        useGrouping: true
    });

const balanceCheckbook = (checkbook) => {
    const [openingBalanceStr, ...entries] = 
        sanitize(checkbook)
        .trim()
        .split("\n")
        .filter(notBlank);

    const openingBalance = Number(openingBalanceStr);

    const initialState = { 
            openingBalance, 
            entries: [], 
            averageExpense: 0, 
            totalExpenses: 0
    };

    const report = 
        entries
        .map(extract)
        .sort(byLineAndCategory)
        .reduce( balanceReducer, initialState );

    return `
Original Balance: ${round2(report.openingBalance)}
${report.entries.map(({line, category, expense, balance}) =>
`${line} ${category} ${round2(expense)} Balance ${round2(balance)}`
).join("\n")}
Total Expenses: ${round2(report.totalExpenses)}
Average Expense: ${round2(report.averageExpense)}
`.trim();
};

const tests = () => {
    const assert = (fn, expect) => {
        try {
            const r = fn();
            if (r !== expect) throw new Error(`Expected\n"${expect}"\nGot\n"${r}"`);
        } catch(err) {
            throw new Error(`${err.message}`);
        }
    };
    // check average
    assert( () => balanceCheckbook(`
100.00
100 EntryOne 50.00
101 EntryTwo 50.00
`), `Original Balance: 100.00
100 EntryOne 50.00 Balance 50.00
101 EntryTwo 50.00 Balance 0.00
Total Expenses: 100.00
Average Expense: 50.00`
    );

    // blank lines?
    assert( () => balanceCheckbook(`
100.00
100 EntryOne 50.00

101 EntryTwo 50.00
`), `Original Balance: 100.00
100 EntryOne 50.00 Balance 50.00
101 EntryTwo 50.00 Balance 0.00
Total Expenses: 100.00
Average Expense: 50.00`
    );

    // Random characters?
    assert( () => balanceCheckbook(`
100.00
100 EntryOne#() 50.00

101 EntryTwo;_ 50.00
`), `Original Balance: 100.00
100 EntryOne 50.00 Balance 50.00
101 EntryTwo 50.00 Balance 0.00
Total Expenses: 100.00
Average Expense: 50.00`
    );

    // what if there are no entries?
    assert( () => balanceCheckbook(`
100.00
`), `Original Balance: 100.00

Total Expenses: 0.00
Average Expense: 0.00`
    );

    // negative?
    assert( () => balanceCheckbook(`
1000.00
100 Groceries 79.99
103 Transmission 4,999.39
`), `Original Balance: 1,000.00
100 Groceries 79.99 Balance 920.01
103 Transmission 4,999.39 Balance -4,079.38
Total Expenses: 5,079.38
Average Expense: 2,539.69`
    );
}
tests();

// example checkbook
const checkbook = `
1233.00
125 Hardware;! 24.8?;
123 Flowers 93.5
127 Meat 120.90
120 Picture 34.00
124 Gasoline 11.00
123 Photos;! 71.4?;

122 Picture 93.5
132 Tires;! 19.00,?;
129 Stamps 13.6
129 Fruits{} 17.6
129 Market;! 128.00?;
121 Gasoline;! 13.6?;
`;

console.log(balanceCheckbook(checkbook));