DOMContentLoaded vs load

Differences

The DOMContentLoaded event fires when all the nodes in the page have been constructed in the DOM tree.
The load event fires when all resources such as images and sub-frames are loaded completely.

Good to know

The DOMContentLoaded event isn't supported in IE 8 and the older versions of IE.

To support the behavior in the old versions of IE, we can use the readyState property to check if the document is loaded completely or not:

const ready = function (cb) {
    document.readyState === 'loading'
    // The document is still loading
    ? document.addEventListener('DOMContentLoaded', function (e) {
        cb();
    })
    // The document is loaded completely
    : cb();
};

The ready function accepts a function that will be invoked when the document is ready:

ready(function() {
    // Do something when the document is ready
    ...
});