While writing JavaScript beginners get a lot of Uncaught TypeError: Cannot set property ‘…’ of null error message so what is the problem with the code?

Define Object During JavaScript Initialization

Here is number one reason the object you try to access is not defined. It may define later. It is not a joke because if you are using JavaScript framework some java objects can be defined later. So for example if you are using jQuery following document ready function can help.

$(document).ready(function() {
    //Javascript code
}