How to check if function exists in javascript

Javascript might not be happy if you call function before it is being created. This can happen in a couple of cases.

You might be wondering how to check if function exists in javascript before calling it.

Lets say you have a onLoad logic, and there could be function definition below it. In that case, if you try to call the function from the onLoad, it might complain as

Uncaught referenceError: functionName is not defined 

This happens only because the browser will be creating the function later and there is no precedence on this.

How to fix Uncaught referenceError: is not defined javascript error

In this case you can check if the function exists first and call it afterwards:

if (typeof functionName == 'function') {
    functionName();
}

This will solve the problem. But there is a catch, you need to recall it again you make sure it is loaded.

Let me know if this solves your problem or not. I can add more based on your questions or suggestions.