First to answer your question:

Edit: Nowadays JavaScript does have a startsWith function on the String object.

JavaScript does not have a 'StartsWith' method on the String object. You will have to use regular expressions for this. Here's an example:

function validate() {

    var email = document.getElementById('email').value;

    var phone = document.getElementById('phone').value;



    var emailFilter = /^([a-zA-Z0-9_.-])+@(([a-zA-Z0-9-])+.)+([a-zA-Z0-9]{2,4})+$/;

    var phoneFilter = /^http:///;



    if (!emailFilter.test(email)) {

        alert('Please enter a valid e-mail address.');

        return false;

    }



    if (!phoneFilter.test(phone)) {

        alert('Please correct your phone number.');

        return false;

    }



    return true;

}

I created a jsfiddle to show you how it works: http://jsfiddle.net/cnVQe/1/

However: it's best to use a declarative instead of a programmatic approach for validating forms. There are many tools that do this. One of them is this jQuery library that does most of the heavy listing for you: http://jqueryvalidation.org/

It works by just adding classes to to the fields and have the plugin do the checking , error displaying and preventing of form submission.

Just to add: you approach is not wrong, but if the form gets bigger and you want to check more fields you will inevitably run into complexity that has been solved over and over by other people. Even if it means that you pull in additional complexity with the extra plugin it will all be worth it as soon as you start to make the form bigger.

Also: your regular expression for the e-mail address check will not accept the + sign, which is something many people use to create special gmail accounts like [email protected]. Using libraries such as the one I describes very often have robust checks already. So that's an additional benefit on using existing libraries.