JavaScript Application Cookbook

numbers.js

Practical uses

Correcting rounding errors and number formatting for shopping cart programs

Version requirement

JavaScript 1.1

Functions

twoPlaces(), round(), totals()

JavaScript performs floating point arithmetic a little differently than we might expect; the result is that many of our calculations also come out slightly different than what we expect. The DevEdge Newsgroup FAQ at http://developer1.netscape.com:80/support/faqs/champions/javascript.html#2-2 uses the example 0.119 * 100 = 11.899999. It’s also common to want to display number in terms of dollars and cents. The functions in numbers.js are designed to help with both situations. All three functions included are based on those at http://www.irt.org/script/number.htm maintained by Martin Webb. Figure 6.14 shows ch06\numbers.html loaded.

Figure 6-14. Better looking numbers, thanks to JavaScript

The numbers displayed under the “Two Decimal Places” heading shows how function twoPlaces() formats numbers to place them in a dollars and cents format. The other two headings display the difference between the expression 51.02 - 3.8 both without and with function round() and totals(), the latter of which is the desired effect. Example 6.9 shows numbers.js.

Example 6-9. number.js

 1 function twoPlaces(amount) { 2 return (amount == Math.floor(amount)) ? amount + '.00' : 3 ((amount*10 == Math.floor(amount*10)) ? ...