Validating form input

Validating form input

Form input is very important for some Web- pages. The form input is often sent back to the server again. JavaScript has got the functionality to validate the form input before sending it to the server. First I want to show you how forms can be validated. Then we will have a look at the possibilties for sending information back with JavaScript or HTML.

First of all we want to create a simple script. The HTML- page shall contain two text- elements. The user has to write his name into the first and an e-mail address into the second element. You can try this scripts in the online version of my tutorial.

Concerning the first input element you will receive an error message when not entering anything. Any input is seen as valid input. Of course, this does not prevent the user from entering any wrong name. The browser even accepts numbers. So if you enter '17' you will get 'Hi 17!'.

The second form is a little bit more sophisticated. If you try to enter a simple string - your name for example it won't work (unless you have a @ in your name...). The criteria for accepting the input as a valid e-mail address is the @. A single @ will do it - but this is certainly not very meaningful. Every Internet e-mail address contains a @ so it seems appropriate to check for a @ here.

What does the script for those two form elements and for the validating look like? Here it goes:

Enter your name:

Enter your e-mail address:

First have a look at the HTML- code in the body- section. We just create two text elements and two buttons. The buttons call the functions test1(...) or test2(...) depending on which button is pressed. We pass this.form to the functions in order to be able to address the right elements in the functions later on. The function test1(form) tests if the string is empty. This is done via if (form.text1.value == "")... . 'form' is the variable which receives the 'this.form' value in the function call. We can get the value of the input element through using 'value' in combination with form.text1. In order to look if the string is empty we compare it with "". If the input string equals "" then no input was done. The user will get an error message. If something is entered the user will get an ok.

The problem here is that the user might enter only spaces. This is seen as a valid input! If you want to, you can of course check for these possibilities and exclude them. I think this is quite easy with the information given here.

Now have a look at the test2(form) function. This function again compares the input string with the empty string "" to make sure that something has been entered. But we have added something to the if- command. The || is called the OR- operator. You have learned about it in part 6 of this introduction. The if- command checks if either the first or the second comparison is true. If at least one of them is true the whole if- command gets true and the following command will be executed. This means that you will get an error message either if your string is empty or if there isn't a @ in your string. The second operation in the if- command looks if the entered string contains a @.