GetQueryStrings.js

const queryStrings = window.location.search.substring(1).split("&").reduce((output, item, index) => {
    if(item === "")
        return output;
    
    const keyValue = item.split("=");
    
    output[decodeURIComponent(keyValue[0])] = keyValue[1] !== undefined ?
        decodeURIComponent(keyValue[1]) :
        true; // return true for query strings with no value
    
    return output;
}, {});

function getQueryString(key) {
    const keyMatch = Object.keys(queryStrings).filter(item => item.match(new RegExp(`^${key}$`, "i")))[0];
    return keyMatch !== undefined ? queryStrings[keyMatch] : undefined;
}