Add a rounding method to Number.prototype in JavaScript

Question

How can I simplify rounding in JavaScript? I wish that I could do it in a more elegantly in an object-oriented manner. The method toFixed works well, but does not have backward rounding and it also returns a string and not a number.

pi.toFixed(2).valueOf();
// 3.14

As it is, rounding is a bit of a tangle because I have to use:

pi = Math.round(pi * 100) / 100;
// 3.14

It would be much nicer instead just to stick a method to the end of a variable, such as:

pi.round(2);
// 3.1r

Answer 1

Extend Number.prototype. Numbers in Javascript are a data type that is associated with the built-in object "Number." Add the following polyfill block:

if (!Number.prototype.round) {
    Number.prototype.round = function (decimals) {
        if (typeof decimals === 'undefined') {
            decimals = 0;
        }
        return Math.round(
            this * Math.pow(10, decimals)
        ) / Math.pow(10, decimals);
    };
}

Anywhere after this, you can round numbers by sticking .round() to the end of them. It has one optional parameter that determines the number of decimals. For example:

pi.round(2);

You can also use backward rounding with negative numbers such as:

x = 54321;
x.round(-4);
// 50000

Fiddle: http://jsfiddle.net/g2n2fbmq/

Related:

• Javascript Convert numbers to different formats or string alternative
• Extending Number.prototype in javascript and the Math object?
• Integer prototype