How to check if function exists in JavaScript?

Question

My code is

function getID( swfID ){      if(navigator.appName.indexOf("Microsoft") != -1){           me = window[swfID];      }else{           me = document[swfID];      } }  function js_to_as( str ){      me.onChange(str); } 

However, sometimes my onChange does not load. Firebug errors with

me.onChange is not a function

I want to degrade gracefully because this is not the most important feature in my program. typeof gives the same error.

Any suggestions on how to make sure that it exists and then only execute onChange?

(None of the methods below except try catch one work)

Answer 1

Try something like this:

if (typeof me.onChange !== "undefined") {      // safe to use the function } 

or better yet (as per UpTheCreek upvoted comment)

if (typeof me.onChange === "function") {      // safe to use the function } 

Answer 2

I had this problem.

if (obj && typeof obj === 'function') { ... } 

kept throwing a reference error if obj happened to be undefined.

In the end I did the following:

if (typeof obj !== 'undefined' && typeof obj === 'function') { ... } 

A colleague pointed out to me that checking if it's !== 'undefined' and then === 'function' is redundant of course.

Simpler:

if (typeof obj === 'function') { ... } 

Much cleaner and works great.