On large applications, there may be some instances where you need to make a call to a function which belongs in a different javascript file.

There might also be a case where this javascript file isn’t always included on every page, and should only be called if it is included.

If we are to call this function, and it hasn’t been included, a javascript error will be returned.

Therefore we need to first check that the functions exists in the current context. Using typeof we can determine whether the browser detects the function name as a function, and run it only if it exists:

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