One-statement parsing location.parameters

golf-location-parameters.js

decodeURIParam=p=>//<223 bytes w/o whitespace and comments
Object.assign(//Create a new object,
 ...//the union of the following objects:
 p//location.parameters
 .match(/^\??(.*)$/)//has a leading '?'
 [1]//(which we don't want)
 .split('&')//split the rest into '&'-delimited chunks,
 .filter(S=>S)//remove null chunks (from duplicate or trailing ampersands)
 .map(S=>S//take each chunk
  .match(/^([^=]*)=?(.*)$/))//and split it via key=value
 .map(([S,k,v])=>(//take each of those key,value pairs
  {//put them into objects
   [decodeURIComponent(k.replace(/\+/g,' '))]//(of course, only after
   :decodeURIComponent(v.replace(/\+/g,' '))})))// decoding them)
;//call via decodeURIParam(location.parameters)

golf-location-parameters.min.js

decodeURIParam=p=>Object.assign(...p.match(/^\??(.*)$/)[1].split('&').filter(S=>S).map(S=>S.match(/^([^=]*)=?(.*)$/)).map(([S,k,v])=>({[decodeURIComponent(k.replace(/\+/g,' '))]:decodeURIComponent(v.replace(/\+/g,' '))})));