uCertify CIW – Advanced HTML5 & CSS3 Specialist Practice Test

Functions must be defined and loaded before any user action can invoke them. Placing the code that defines your functions in the head of the HTML page ensures those functions are parsed and available as soon as the page starts loading, so when the user interacts with the page later, the functions are already defined and ready to run. This early loading avoids timing issues where a click or other event tries to call a function that hasn’t been defined yet.

Putting functions at the end of the body can work in many cases, but it’s not as robust. If something causes a user action to occur before that script finishes loading or executing, the function won’t exist yet and an error will occur. Inline event handlers depend on the function being available at the moment the event fires, so they share the same risk if script loading is delayed. Loading an external script asynchronously introduces a race condition: the script might still be loading when the user triggers an event, so the function may not be defined when needed.

So defining all your functions in the head makes their availability guaranteed before any events can call them.