If you want to quickly access the value of an HTML input give it an id to make your life a lot easier. This small script below will check to see if there is any text in the text field "myText". The argument that getElementById requires is the id of the HTML element you wish to utilize.
var myTextField = document.getElementById('myText');
if(myTextField.value != "")
alert("You entered: " + myTextField.value)
alert("Would you please enter some text?")
<input type='text' id='myText' />
<input type='button' onclick='notEmpty()' value='Form Checker' />
document.getElementById returned a reference to our HTML element myText. We stored this reference into a variable, myTextField, and then used the value property that all input elements have to use to grab the value the user enters.
There are other ways to accomplish what the above script does, but this is definitely a straight-forward and browser-compatible approach.
Things to Remember About getElementById
When using the getElementById function, you need to remember a few things to ensure that everything goes smoothly. You always need to remember that getElementById is a method (or function) of the document object. This means you can only access it by using document.getElementById.
Also, be sure that you set your HTML elements' id attributes if you want to be able to use this function. Without an id, you'll be dead in the water.
If you want to access the text within a non-input HTML element, then you are going to have to use the innerHTML property instead of value. The next lesson goes into more detail about the uses of innerHTML.