Autofill forms from url query

autofill_form.js

(function() {
  
  // These are the form fileds you want to fill
  // All You have to do is define the classname of the field and url parameter you want to fill, it with
  var formFields = [
    { className: "field1", parameter: "value1" },
    { className: "field2", parameter: "value2" },
    { className: "field3", parameter: "value3" }
  ];

  var field;

  // Here an object is created containing all available parameters from url string
  var queryString = function () {
    var query_string = {},
        query = window.location.search.substring(1),
        vars = query.split("&"),
        arr, pair;
    for (var i = 0; i < vars.length; i++) {
      pair = vars[i].split("=");
      if (typeof query_string[pair[0]] === "undefined") {
        query_string[pair[0]] = pair[1];
      } else if (typeof query_string[pair[0]] === "string") {
        arr = [query_string[pair[0]], pair[1]];
        query_string[pair[0]] = arr;
      } else {
        query_string[pair[0]].push(pair[1]);
      }
    }
    return query_string;
  }();

  // Loops through formFields and checks if element with className exists and if parameter exists in queryString
  for (var f = 0, max = formFields.length; f < max; f++) {
    field = document.querySelector("." + formFields[f].className + " input, ." + formFields[f].className + ' textarea');
    if (field && typeof queryString[formFields[f].parameter] !== "undefined") {
      field.value = decodeURIComponent(queryString[formFields[f].parameter]);
    }
  }

})();