What's the point of using this onLoad function from "JavaScript: The Definite Guide"?

Question

The code snippet below is from the book JavaScript: The Definite Guide.

I have some questions about it. From my point of view, I don't anticipate any scenario where there is a must to use the onLoad function.

I think that this chunk of code has to be used as global javascript instead of as part of an event handler. If it is invoked in an event handler of a button, the load event on window must have already been triggered. As a result, the registered function will never be invoked.

However, if it is used as the global javascript, onLoad.loaded is always false when the onLoad function is invoked.

Why not just register the load event on window, instead of checking whether onLoad.loaded is true or false?

// Register the function f to run when the document finishes loading.
// If the document has already loaded, run it asynchronously ASAP.
function onLoad(f) {
    if (onLoad.loaded) // If document is already loaded
        window.setTimeout(f, 0); // Queue f to be run as soon as possible
    else if (window.addEventListener) // Standard event registration method
        window.addEventListener("load", f, false);
    else if (window.attachEvent) // IE8 and earlier use this instead
        window.attachEvent("onload", f);
}

// Start by setting a flag that indicates that the document is not loaded yet.
onLoad.loaded = false;

// And register a function to set the flag when the document does load.
onLoad(function() {
    onLoad.loaded = true;
});

Answer 1

I think you misunderstand the usage scenario.

You are right, the above snippet should be executed on startup, before the webpage completes loading, so that onLoad.loaded is accurately assigned.

However, the onLoad function that is defined by the script can be invoked from anywhere, later, even from button event handlers etc.