avoid rounding errors in javascript with bigdecimal?

Question

a user submits a buy request: 0.0104 BTC at a rate of 345.92 EUR

config:

var mode = RoundingMode.HALF_EVEN();
var satoshi = new BigDecimal("100000000");

user input:

var btc_b = new BigDecimal("0.0104");
var rate_b = new BigDecimal("345.92");

convert amount to smallest unit:

var btc_s = btc_b.multiply(satoshi);

btc * rate = spend:

var eur_s = btc_s.multiply(rate_b).setScale(0,mode);
console.log(eur_s.longValue());

output: 359756800

eur / smallest unit:

var eur_b = eur_s.divide(satoshi, 8, mode);
console.log(eur_b.longValue());

output: 3.597568

but since eur has only 2 decimals:

console.log(eur_b.setScale(2,mode).longValue());

output: 3.6

i assume the user must spend 3.6 EUR for 0.0104 BTC at a rate of 345.92 EUR

here's a fiddle: jsfiddle.net/nvkja98c/

please correct me if i'm wrong!

Answer 1

This is not a rounding problem. The 3.6 EUR is correctly rounded to two decimals from 3.597568 EUR.

Computers don't know when we want to see trailing zeros. The problem is formatting:

console.log(eur_b.setScale(2,mode).longValue().toFixed(2));

output: 3.60

(fiddle)