window.location.search query as JSON

Here's a pure JS function. Parses the search part of the current URL and returns an object. (It's a bit verbose for readability, mind.)

function searchToObject() {
  var pairs = window.location.search.substring(1).split("&"),
    obj = {},
    pair,
    i;

  for ( i in pairs ) {
    if ( pairs[i] === "" ) continue;

    pair = pairs[i].split("=");
    obj[ decodeURIComponent( pair[0] ) ] = decodeURIComponent( pair[1] );
  }

  return obj;
}

On a related note, you're not trying to store the single parameters in "a JSON" but in "an object". ;)

Please also note that there's an api to query/manipulate search params with: https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams

var params = new URLSearchParams(window.location.search)
for (let p of params) {
  console.log(p);
}