init.js

const init = new function () {
    let funcs = [],
        booted = 0,
        doc = document,
        ready = fn => {
            'loading' === doc.readyState ?
                doc.addEventListener('DOMContentLoaded', fn) :
                fn();
        };

    this.push = fn => {
        booted ? ready(fn) : funcs.push(fn);
    };

    this.boot = () => {
        if (!booted) {
            booted = 1;
            funcs.forEach(fn => ready(fn));
        }
    };
};

/* minified */
const init=new function(f=[],b=0,d=document,r=c=>"loading"===d.readyState?d.addEventListener("DOMContentLoaded",c):c()){this.push=c=>{b?r(c):f.push(c)};this.boot=_=>{if(!b){b=1;f.forEach(c=>r(c))}}};
/* jQuery version */
const init=new function(f=[],b=0){this.push=c=>{b?$(c):f.push(c)};this.boot=_=>{if(!b){b=1;f.forEach(c=>$(c))}}};