How can I get the minor currency unit with rounding?

[DustinJSilk]

After doing a few calculations, I've found my scale is larger than the currencies original scale.
This means that when I create a snapshot to get the amount, the amount doesn't actually represent the currencies minor value. And the toUnit method can only safely return a major unit.

For example:

const cost = dinero({ amount: 1000, currency: USD });
const taxed = multiply(cost, { amount: 11256, scale: 4 });

const {
  amount, // 11256000
  scale, // 6
  currency,
} = toSnapshot(taxed);

In this case, I'd need to send stripe 1126, not 112560.
If I use transformScale, I get 1125, not 1126.

This is the only way I could figure it out:

const unit = toUnit(taxed, { digits: currency.exponent, round: up });
const minorFromUnit = unit * Math.pow(currency.base, currency.exponent)
console.log(minorFromUnit) // 1126

A toMinorUnit would be extremely helpful in this type of situation.
Or potentially giving the toUnit function a scale option.
Or even give a rounding option to transformScale. This way we can easily save snapshots in a database without worrying about saving an amount that could be different to the amount charged to customers.

Appreciate the effort!
Thanks

[tranhl]

@DustinJSilk FYI the solution you provided is potentially erroneous as the float multiplication without rounding can create imprecise values (e.g. 6581.99999999), so you'll have to apply a rounding mode after the multiplication:

const minorFromUnit = halfUp(unit * Math.pow(currency.base, currency.exponent))

[sarahdayan]

Hey @DustinJSilk and sorry for the late reply.

You're right, there currently isn't a good way to transform an object with loss and specify how the loss is handled—transformScale just truncates the amount. I've thought about it for a while, and I'm leaning towards providing a userland rounding function to transformScale.

If you'd like to try it out, a PR is always welcome!

This way we can easily save snapshots in a database without worrying about saving an amount that could be different to the amount charged to customers.

I would generally recommend persisting accurate, precise snapshots, and saving rounded amounts separately. Whenever you need to do further calculations, it's usually nice to have the raw, unatered data saved somewhere.