Uncaught TypeError: Cannot read property 'className' of undefined

Question

The following error is being thrown in Google Chrome\'s developers tools:

Uncaught TypeError: Cannot read property \'className\' of undefined

Answer 1

It's throwing this error because when dom first loads you were using before declare.

for this error i had one solution written below:

you can add if condition particular element is not undefined like below

if(document.getElementsByTagName('button') !== undefined){
  // and write your needed code here
}