When a web page is ready for manipulation. Altrernative for jQuery ready()

ready.js

// Invokes callback when DOM is ready for manipulation
// Usage: ready(function () {});

function ready(callback) {
    // Motivation:
    // https://gomakethings.com/a-native-javascript-equivalent-of-jquerys-ready-method/
    // Docs:
    // https://developer.mozilla.org/en/docs/Web/API/Document/readyState
    var loading, done;

    done = function () {
        document.removeEventListener('readystatechange', loading);
        window.removeEventListener('load', done);
        callback();
    };

    loading = function () {
        if (document.readyState === 'loading') {
            return true;
        }
        done();
    };

    if (loading()) {
        document.addEventListener('readystatechange', loading);
        window.addEventListener('load', done);
    }
};