Why is my 'load' event/function not beeing executed after switching to jQuery 3?

The problem can be occur when using/switching to jQuery 3. It's because all ready states in the new jQuery 3 are now fully asynchron. This means, that there is no given order for your code to be executed.

Because of this, it could happen, that load is been triggered before your ready state has been executed. When your ready function now finally gets triggered, your load listener is too late and will not be executed.

jQuery Usage:

To change this behavior, just remove the ready state around your load event listener initialization. There is no need to encapsulate this with a ready function. You can initialize them without.

// $(function() {
    $(window).on("load", function() {
        // this line will now be executed again
        console.log("window is loaded!");
    });
// });

If you need or want to register both events, you can register the load event by yourself and decide inside the ready state what to do next.

// track the loading state by yourself
var windowLoaded = false;
$(window).on("load", function() {
    windowLoaded = true;
});

$(function() {
    function afterLoad() {
        console.log("loaded");
    }

    // decide inside your ready state what to do
    if( !windowLoaded ) {
        $(window).on("load", afterLoad);
    }
    else {
        afterLoad();
    }
});

jQuery Plugins:

Another case would be jQuery plugins, that uses the load event too. For example:

(function($, window) {
    $.fn.myPlugin = function() {
        $(window).on("load", start);

        function start() {
            console.log("plugin initialized and window loaded");
        }
    };
})(jQuery, window);

If a developer/user now wraps the plugin initialization in a ready state the problem could happen again, just like explained before:

$(function() {
    $("#element").myPlugin();
});

A solution would be to track the load event in your plugin on your own, to break out the ready state.

(function($, window) {
    // track the loading state beside the plugin initialization
    var windowLoaded = false;
    $(window).on("load", function() {
        windowLoaded = true;
    });

    $.fn.myPlugin = function() {
        // decide inside your plugin how to start
        if( !windowLoaded ) {
            $(window).on("load", start);
        }
        else {
            start();
        }

        function start() {
            console.log("plugin initialized and window loaded");
        }
    };
})(jQuery, window);

Conclusion:

Even when this problem not happens to you, in your tests, you should change those code immediately, when using jQuery 3, because other users/different browser can run into this trouble. Others may got the problem, because it is asynchron, you could never know when/if your code gets executed ...