Using reduce to get a sum of object values, but the answer is a few decimals short (JS)

Question

I have the following code:

function checkCashRegister(price, cash, cid) {

    // Move cid into an object
    var cashReg = cid.reduce(function(prev, curr) {
        prev[curr[0]] = curr[1];
        return prev;
    }, {});

    // Total money in the register
    var regSum = Object.values(cashReg).reduce(function(a, b) {
        return a + b;
    }, 0);

    return regSum;
    // --> 335,41
}

checkCashRegister(19.50, 20.00, [
    ["PENNY", 1.01],
    ["NICKEL", 2.05],
    ["DIME", 3.10],
    ["QUARTER", 4.25],
    ["ONE", 90.00],
    ["FIVE", 55.00],
    ["TEN", 20.00],
    ["TWENTY", 60.00],
    ["ONE HUNDRED", 100.00]
]);

The code itself seems to be working just fine, but for some reason when I return regSum the answer is 335,40999999999997 instead of 335,41. Why does this happen and how could I change my code so it would return the expected outcome?

score 2 · Accepted Answer · answered Mar 01 '18 at 03:00

Just as Geuis sayed it is the number precision problem. If you expected the outcome only retains two digits after the decimal point, you can change "return regSum;" to "return Number(regSum.toFixed);". function checkCashRegister(price, cash, cid) {

// Move cid into an object
var cashReg = cid.reduce(function(prev, curr) {
    prev[curr[0]] = curr[1];
    return prev;
}, {});

// Total money in the register
var regSum = Object.values(cashReg).reduce(function(a, b) {
    return a + b;
}, 0);

return Number(regSum.toFixed(2));
// --> 335,41

}

score 0 · Answer 2 · answered Mar 01 '18 at 02:24

0

You're running into the javascript number precision problem. Read more here How to deal with floating point number precision in JavaScript?

answered Mar 01 '18 at 02:24

Geuis

41,122
56
157
219

score 0 · Answer 3 · answered Mar 01 '18 at 02:40

A good way to get around this is to scale you numbers. So for $1.50, you could have 150(cents), and do your math based on that, and remember to multiply by 0.01 before displaying. I know that may be a pain, but that's javascript. If it's going to be too much of a change to do that, you could always use a library, such as decimal.js. You can read the docs here.

Using reduce to get a sum of object values, but the answer is a few decimals short (JS)

3 Answers3