Javascript Error Null is not an Object

Question

Im getting a error in the Web Inspector as shown below:

TypeError: 'null' is not an object (evaluating 'myButton.onclick = function() { var userName = myTextfield.value; greetUser(userName);  return false;  }') 

Here is my Code (HTML):

<h2>Hello World!</h2> <p id="myParagraph">This is an example website</p>  <script src="js/script.js" type="text/javascript"></script>  <form>   <input type="text" id="myTextfield" placeholder="Type your name" />   <input type="submit" id="myButton" value="Go" /> </form> 

Here is the JS:

var myButton = document.getElementById("myButton"); var myTextfield = document.getElementById("myTextfield");  function greetUser(userName) { var greeting = "Hello " + userName + "!"; document.getElementsByTagName ("h2")[0].innerHTML = greeting; }  myButton.onclick = function() { var userName = myTextfield.value; greetUser(userName);  return false;  } 

Any Idea why I am getting the error?

Answer 1

Put the code so it executes after the elements are defined, either with a DOM ready callback or place the source under the elements in the HTML.

document.getElementById() returns null if the element couldn't be found. Property assignment can only occur on objects. null is not an object (contrary to what typeof says).

Answer 2

Any JS code which executes and deals with DOM elements should execute after the DOM elements have been created. JS code is interpreted from top to down as layed out in the HTML. So, if there is a tag before the DOM elements, the JS code within script tag will execute as the browser parses the HTML page.

So, in your case, you can put your DOM interacting code inside a function so that only function is defined but not executed.

Then you can add an event listener for document load to execute the function.

That will give you something like:

<script>   function init() {     var myButton = document.getElementById("myButton");     var myTextfield = document.getElementById("myTextfield");     myButton.onclick = function() {       var userName = myTextfield.value;       greetUser(userName);     }   }    function greetUser(userName) {     var greeting = "Hello " + userName + "!";     document.getElementsByTagName ("h2")[0].innerHTML = greeting;   }    document.addEventListener('readystatechange', function() {     if (document.readyState === "complete") {       init();     }   });  </script> <h2>Hello World!</h2> <p id="myParagraph">This is an example website</p>  <form>   <input type="text" id="myTextfield" placeholder="Type your name" />   <input type="button" id="myButton" value="Go" /> </form> 

Fiddle at - http://jsfiddle.net/poonia/qQMEg/4/