SPEAKER: Let's now take a look at a web page that allows a user to register for something but that actually includes some client-side validation of his or her inputs. 

Notice here, in form-1.HTML, I have that same form as before but I've added an ID attribute to my form tag, the value of which is register, so that I have an ID via which I can uniquely identify that form in my DOM. Now notice below my form tag is a script tag, so that very deliberately, my JavaScript code will only execute once the form tag and its DOM nodes have been loaded. 

Inside of the script tag notice is the first line here where I declare a variable called form and assign it the return value of document.getElementByID of quote unquote registration. It's this line of code that will look through my DOM, looking for the element or node that has a unique identifier of registration, storing the return value ultimately in this variable. 

Then I register with that form an event handler for the form submission, with form.onsubmit and assign that an anonymous function, the body of which proceeds to do the following. If that form's field, whose name is email, has a value from the user equal to quote unquote nothing, then we're going to alert the user that he or she must provide their email address, and we return false so that the form itself is not submitted to register dot php. 

Else, if the form has a field whose name is password and whose value is quote unquote, then let's yell at the user that he or she must provide a password. And again, return false so that the form is not submitted to register dot php. 

Meanwhile, if the value the user has typed in to the form field called password does not match the value that the user has provided for the form field called confirmation, then let's yell at the user the passwords do not match, and then return false so that the form is not submitted to register dot php. 

Lastly, if it is not the case that the form's agreement input is checked, then let's yell at the user explaining that he or she must agree to the terms and condition, and again return false so that the form is not submitted to register dot php. 

Else if, none of those mistakes have been made, let's indeed return true and allow the form to be submitted to register dot php. 

Let's take a look at these possible errors now by opening the page in a browser. http://localhost/form-1.HTML. Here's then that form. Let's provide nothing. 

You must provide your email address. All right, let's at least cooperate along those lines. 

You must provide a password. All right, let's choose a password like crimson. Passwords do not match. Ah, I need to cooperate and provide the same word, crimson, again. 

You must agree to the terms and conditions. All right, let's now check that box. And finally, whew, I'm registered.