How to check if a variable or object is undefined?

Question

I always thought I could just check an undefined var by comparing it to undefined, but this is the error I get in the chrome console :

Answer 1

You can use : if( typeof jQuery !== 'undefined')

or

Do what is recommended by mozilla

if('jQuery' in window)

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/undefined