The purpose of a form validation script is to return a boolean value ('s to reference form fields, but that can lead to namespace conflicts and why make things more complicated than necessary.When the form is submitted - either by hitting Enter or clicking on the Submit button - the (the 'value' of the field called 'input' belonging to the form). In a real-life situation you will most likely have more fields to check, and more complicated conditions, but the principle remains the same.Other form values are available using a similar syntax, although this becomes more complicated if you're using SELECT lists, checkboxes or radio buttons (see below for examples). All you need to do is extend the command is encountered, execution of the function is halted.In other words if the first condition fails, the second condition will not be tested and so forth.Only when all conditions have been satisfied do we reach the command, in which case the form will be submitted.
Otherwise a browser with Java Script disabled, or a hacker trying to compromise your site, can easily submit invalid data. The first test in the example is therefore only necessary in order to provide a different error message when the input is blank.The illustration below shows this relationship: Note that the 'I' in selected Index needs to be capitalised - Java Script functions and variables are always case-sensitive. Read more about the humble checkbox in our HTML5 Checkbox Validation article.Radio buttons are implemented as if they were an array of checkboxes.Listing 3: Validation for all letters Figure 3: Above output checks the entered data is letters or include any other char also. Two of them are using regular expression for matching and the other one is simple so not writing regex but if you want you can write a regex for this case also.There are various validation schemes for checking the other type of validations, like check for all or no, checking for all letters and numbers and restricting the length of field.