rounding.js

Exact rounding to a specified number of decimal places with a choice of rounding algorithms

Usage

toFixed produces incorrect output for a large number of inputs. In contrast, the roundTo method provided by rounding.js implements an algorithm that produces correct and exact results. To compare:

(1.015).toFixed(2); // 1.01
(1.015).roundTo(2); // 1.02

Unlike toFixed, roundTo handles arbitrarily large and small inputs:

(1.15e-26).toFixed(27); // browser-dependent; an exception is likely
(1.15e-26).roundTo(27); // 0.000000000000000000000000012

rounding.js offers a choice of algorithms. The default is round half to even, but this behaviour can be overridden by setting the Rounding.algorithm property:

// Default algorithm is Rounding.HALF_TO_EVEN
(1.25e-26).roundTo(27); // 0.000000000000000000000000012
Rounding.algorithm = Rounding.HALF_TO_PLUS_INFINITY
(1.25e-26).roundTo(27); // 0.000000000000000000000000013

Rounding.HALF_TO_PLUS_INFINITY is what Math.round uses. To get output similar to toFixed set the algorithm to Rounding.HALF_AWAY_FROM_ZERO

By default, roundTo suppresses trailing zeroes. You can override this with a second parameter specifying the minimum number of fractional digits:

(1.1).roundTo(2);    // 1.1
(1.1).roundTo(2, 2); // 1.10