let callbacks = [];

const dynamicLoadScript = (src, callback) => {
    const existingScript = document.getElementById(src);
    const cb = callback || function() {};

    if (!existingScript) {
        const script = document.createElement('script');
        script.src = src; // src url for the third-party library being loaded.
        script.id = src;
        document.body.appendChild(script);
        callbacks.push(cb);
        const onEnd = 'onload' in script ? stdOnEnd : ieOnEnd;
        onEnd(script);
    } else if (existingScript && cb) {
        cb(null, existingScript);
    }

    function stdOnEnd(script) {
        script.onload = function() {
            // this.onload = null here is necessary
            // because even IE9 works not like others
            this.onerror = this.onload = null;
            for (const cb of callbacks) {
                cb(null, script);
            }
            callbacks = null;
        };
        script.onerror = function() {
            this.onerror = this.onload = null;
            cb(new Error('Failed to load ' + src), script);
        };
    }

    function ieOnEnd(script) {
        script.onreadystatechange = function() {
            if (this.readyState !== 'complete' && this.readyState !== 'loaded') { return; }
            this.onreadystatechange = null;
            for (const cb of callbacks) {
                cb(null, script); // there is no way to catch loading errors in IE8
            }
            callbacks = null;
        };
    }
};

export default dynamicLoadScript;