Javascript Form Error Handling
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: Javascript Post a comment Email Article Print Article Share Articles Reddit Facebook Twitter del.icio.us Digg Slashdot DZone StumbleUpon FriendFeed Furl Newsvine Google LinkedIn MySpace Technorati YahooBuzz Improved Form Handling using JavaScript By Leidago Noabeb Tweet Forms, as we know, are an indispensible part of web development. IT is often at the forefront of collecting data from users. There are very few,
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if any, restrictions on what a user can enter or do with a form. Often you would want the user to see some kind of feedback when working with forms. For example, if user did not enter an email address correctly, you would want this to be known before the data is submitted to the server on its way to a data store. Client-Side Validation While client-side validation is often much faster and provides the user with a very good experience, it is important to note that client-side validation should NOT replace server-side validation. This is mainly because of security among other reasons and also because JavaScript can be turned off on some web browsers. This article will look at the following areas of form development: Form Design Form Validation Error Message Display Designing Your Form This article also assumes basic knowledge of CSS and HTML. How you design your form plays a role in how easy you will be able to identify your form elements. Being able to identify these form elements makes it possible to manipulate those elements leading to form validation.
of code, you can write your own scripts to validate forms I can't give you a complete script, because each form and each check are different. You'll have to use the elements I explain on this page to registration form validation using javascript in html example build your own script. I created an example form and script that you can study to
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get the hang of it. On this page I discuss the limitations of using JavaScript to check a form, then I'll explain the onsubmit event form handling definition handler, followed by the few methods and properties of the form itself. Then it's time for accessing the form elements and the specific syntax for accessing the user defined value of form elements. See also Jeff Howden's excellent article Forms & http://www.htmlgoodies.com/beyond/javascript/improved-form-handling-using-javascript.html JavaScript Living Together in Harmony for some of the most common usability errors and their solutions. Limitations First of all, you should have a clear idea of what happens when a user submits a form that has a JavaScript validation script: The form is checked by a JavaScript like the one described below. If the script finds a mistake the submission is halted here. The user sees an alert and is asked to re-enter some data. If nothing is wrong--or if JavaScript http://www.quirksmode.org/js/forms.html is disabled--the form is sent to the server and is processed by a CGI script. If the CGI script finds a mistake it generates some HTML with an error message and sends it back to the user. In this case the user has to go back to the form, re-enter some values and again submit it. If no mistakes are found, the CGI script does whatever it has to do with the data and directs the user to a Thank You page. As you see, the form is checked for mistakes twice: by the JavaScript and by the CGI script. The CGI check always works, since CGI is server side. The JavaScript check only works when the user has JavaScript enabled. It follows that the CGI check is the most reliable: it always works regardless of what browser is used. Then why use a JavaScript check too? The JavaScript check is very useful in addition to the CGI check because it can catch mistakes before the form is actually sent to the server. Thus the user doesn't have to use his back button to return to the form, something that may cause confusion, and then search for the incorrect form field, which may cause even more confusion. Therefore the JavaScript check is more user friendly than the CGI check. In addition, when you use JavaScript the server doesn't need to spend quite so much time in error handling and is thus a little quicker. Thi
as expected is a good start. Making your programs behave properly when encountering unexpected conditions is where it really gets challenging. ¶ The problematic situations that a program can encounter fall into two categories: http://eloquentjavascript.net/1st_edition/chapter5.html Programmer mistakes and genuine problems. If someone forgets to pass a required argument to a function, that is an example of the first kind of problem. On the other hand, if a program asks the user to enter a name and it gets back an empty string, that is something the programmer can not prevent. ¶ In general, one deals with programmer errors by finding and fixing them, and with genuine errors by error message having the code check for them and perform some suitable action to remedy them (for example, asking for the name again), or at least fail in a well-defined and clean way. ¶ It is important to decide into which of these categories a certain problem falls. For example, consider our old power function:function power(base, exponent) { var result = 1; for (var count = 0; count < exponent; count++) result *= base; in javascript with return result; } ¶ When some geek tries to call power("Rabbit", 4), that is quite obviously a programmer error, but how about power(9, 0.5)? The function can not handle fractional exponents, but, mathematically speaking, raising a number to the halfth power is perfectly reasonable (Math.pow can handle it). In situations where it is not entirely clear what kind of input a function accepts, it is often a good idea to explicitly state the kind of arguments that are acceptable in a comment. ¶ If a function encounters a problem that it can not solve itself, what should it do? In chapter 4 we wrote the function between:function between(string, start, end) { var startAt = string.indexOf(start) + start.length; var endAt = string.indexOf(end, startAt); return string.slice(startAt, endAt); } ¶ If the given start and end do not occur in the string, indexOf will return -1 and this version of between will return a lot of nonsense: between("Your mother!", "{-", "-}") returns "our mother". ¶ When the program is running, and the function is called like that, the code that called it will get a string value, as it expected, and happily continue doing something with it. But the value is wrong, so whatever it ends up doing with it will also be wrong. And if you are unluc