How to tell if a JavaScript function is defined

typeof callback === "function"

if (callback && typeof(callback) == "function")

Note that callback (by itself) evaluates to false if it is undefined, null, 0, or false. Comparing to null is overly specific.

All of the current answers use a literal string, which I prefer to not have in my code if possible - this does not (and provides valuable semantic meaning, to boot):

function isFunction(possibleFunction) {
  return typeof(possibleFunction) === typeof(Function);
}

Personally, I try to reduce the number of strings hanging around in my code...

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Also, while I am aware that typeof is an operator and not a function, there is little harm in using syntax that makes it appear as the latter.