Rounding Decimals in JavaScript

Why it's complicated

JavaScript's Math object provides a method for rounding to whole numbers. If we want to round to a set number of decimal places, then we have to handle that ourselves. This post doesn't get into the details of floating-point arithmetic, but the short of it is most programming languages use a binary floating-point representation which can only approximate many decimal fractions. This results in rounding errors for the most common approaches to rounding in JavaScript.

Rounding Errors

The most common solutions for rounding to a decimal place is to either use Number.prototype.toFixed(), or multiply the float by some power of 10 in order to leverage Math.round(). Both of these work, except sometimes a decimal of 5 is rounded down instead of up.

Number((1.005).toFixed(2)); // 1 instead of 1.01

Math.round(1.005*100)/100; // 1 instead of 1.01

A Better Solution

The rounding problem can be avoided by using numbers represented in exponential notation:

Number(Math.round(1.005+'e2')+'e-2'); // 1.01

And to abstract that into something more usable:

function round(value, decimals) {
  return Number(Math.round(value+'e'+decimals)+'e-'+decimals);
}

round(1.005, 2); // 1.01