Form Calculations

Occasionally, your website might offer some information that the user can customize using a form and involves a small amount of calculation. Maybe the site allows people to calculate the cost of ordering some merchandise or to compute their BMI (body-mass index). Admittedly, this is not common, but if it works for your website, it's a fine JavaScript application. This page demonstrates both of these examples.

Order Form

I recently saw a form similar to the following one on a website offering customized t-shirts. The form did not allow the user to order the merchandise, but it did let them see what they were in for. This form is based on that idea.



Total t-shirts:

Total cost:



Height in :

Weight in :

BMI is , which is

How It's done

A lot of the work is in setting up the forms, and giving an ID to each element that we are interested in: the input elements where we can get numbers and choices from the user and the span elements where we can insert output.

Both forms have an associated global variable that keeps track of the t-shirt cost ($25 for a high-quality shirt and $20 for a regular quality) and the units (centimeters and kilograms versus inches and pounds).

For the order form, whenever the user changes the value of the quality menu, a function named qualityChange is invoked. That function checks the value of the menu and updates the global variable for the t-shirt cost. Similarly, for the BMI form, whenever the user changes the units menu, a function named unitsChange is invoked to deal with that.

In the order form, any time any of the three numbers is changed, a second function, named quantityChange, is invoked. It gets the three numbers out of the form (the number of smalls, the number of mediums and the number of larges) and then recalculates all the costs and totals, inserted the appropriate ones in the document. A simple if statement tests to see whether the user has ordered at least 25 t-shirts and colors the text accordingly. The quantityChange function is added as an event handler to all three inputs using jQuery's .change() method.

In the BMI form, whenever the button is clicked, a function named calcBMI is invoked. That function extracts the two numbers (height and weight) from the form, converts the units to metric if necessary, and calculates the body mass index. Finally, a cascaded if statement categorizes the BMI value and inserts the appropriate text into the document and colors it accordingly.

Use view source on this page to see exactly how we did it.