Parse a URL query i.e. https://example.com/page?queryKey=value&embeddedJson={"json":{"can":"be%20embedded","nested":true}} into an object using javascript. Parses both key=value pairs and nested JSON just fine.

parseQuery.js

// Without an argument, just parse the current page's URL, if an argument is supplied, parse that
function parseQuery(query = window.location.search.substring(1)) {
    // Decode the URL-safe query string into a regular string e.g.("%20" => " ")
    const decodedQuery = decodeURIComponent(query);
    // Split that string at every "&" to get all the key=value pairs
    const allpairs = decodedQuery.split("&");
    // Declare an object to hold our output
    const out = {};
    // This regex matches what seems to look like valid JSON
    const regex = /^[\[|\{](\s|.*|\w)*[\]|\}]$/g;
    for (let i = 0; i < allpairs.length; i++) {
      // Split our key=value pairs
      let pair = allpairs[i].split("=");
      let key = pair[0];
      let val = pair[1];
      // If the value of this pair is matched and determined to be JSON, attempt to parse it
      if (val.match(regex)) {
        // If it is valid JSON then parse it and add it
        try {
          out[key] = JSON.parse(val);
        }
        // If it isn't valid then warn about it and add it as a regular string
        catch (error) {
          console.warn("matched what appeared to be json, failed to parse it");
          out[key] = val;
        }
      }
      // If the value is just a normal, non-json value, add it normally
      else {
        out[key] = val;
      }
    }
  // Return the final output
  return out;
}