[29athrowaway]

I never said you should use floating-point numbers for currency. I said that if you are not aware of the shortcomings of floating-point numbers (the only numeric type in JavaScript, not counting workarounds like typed arrays) you should not be working with values representing currencies until you do.

Fixed-point numbers (aka "decimal" type in Java, C# and others) are the preferred way to deal with this problem as I mentioned in this same discussion hours ago.

Now, in the example you presented, you have a round-off error amplication problem where the round-off error grows larger than the epsilon. You can avoid that using the Kahan summation algorithm.

https://en.wikipedia.org/wiki/Kahan_summation_algorithm

    const arr = [0.1, 0.2, 0.9, 0.6];
    let sum = 0.0;
    let c = 0.0;
    for(let i = 0, l = arr.length; i < l; i++) {
        y = arr[i] - c;
        t = sum + y;
        c = (t - sum) - y;
        sum = t;
    }

Now you evaluate sum, and it's 1.8 as expected.

[29athrowaway]

^ And of course a comment mentioning the correct way of doing things got downvoted.