<form id="target">
  <select name="age">
   <option value="age1">20</option>
   <option value="age2" selected>21</option>
  </select>
  <input name="name" value="Cynthia">
  <label>passsword</label>
  <input type="password" name="password" value="123456">
  <input type="hidden" name="salery" value="3333">
  <textarea name="description" cols="15" rows="5">description</textarea>
  <input type="checkbox" name="hobby" value="football" checked> Football
  <input type="checkbox" name="hobby" value="basketball"> Basketball
  <input type="radio" name="sex" value="Female" checked> Female
  <input type="radio" name="sex" value="Male"> Male
 </form>

function serializeForm1(form){ 
   var setForm = ""; 
   for(var key in form){ 
    if(form.hasOwnProperty(key)){ 
     setForm += '"'+form[key].name+'"'+':'+'"'+form[key].value + '"'+',';
    }
   }
   setForm = "{" + setForm.slice(0,setForm.length -1) + "}";
   console.log(setForm);
   // console.log(JSON.parse(setForm));
   return JSON.parse(setForm);
  }
   
  // 调用
  var oForm = document.getElementById('target');
  console.log(serializeForm3(oForm));

 function serializeForm2(form) {
   var parts = [];
   for (var i = 0, i1 = form.elements.length; i < i1; i++) {
    var field = form.elements[i];
    switch (field.type) {
     case 'select-one':
     case 'select-multiple':
      if (field.type.length) {
       for (var j = 0, j1 = field.options.length; j < j1; j++) {
        var option = field.options[j];
        if (option.selected) {
         var optionValue = '';
         if (option.hasAttribute('value') && option.attributes['value'].specified) {
          //specified表明是否有此属性，有的话返回true，若定义了此属性但尚未添加到元素中也返回true。
          optionValue = option.value;
         } else {
          optionValue = optionValue.text;
         }
         parts.push(encodeURIComponent(field.name) + '=' + encodeURIComponent(optionValue));
        }
       }
      }
      break;
     case undefined:
     case 'file':
     case 'submit':
     case 'reset':
     case 'button':
      break;
     case 'radio':
     case 'checkbox':
      if(!field.checked){
       break;
      }else{
       parts.push(encodeURIComponent(field.name) + '=' + encodeURIComponent(field.dataset['index']));
       break;
      }
     default:
      if(field.name.length){
       parts.push(encodeURIComponent(field.name) + '=' + encodeURIComponent(field.value));
      }
    }
   }
   return parts.join('&');
  }
   
  // 调用
  var oForm = document.getElementById('target');
    console.log(serializeForm2(oForm));

 function serializeForm3(form){
   if(!form||form.tagName.toUpperCase()!='FORM'){
    return false;
   }
   var res=[];
   var tag,tagType,tagVal,tagName;
   for(var i=0;i<form.length;i++){
    tag=form[i];
    tagType=form[i].type;
    tagVal=form[i].value;
    tagName=form[i].name; 
    var reg1=/['textarea'|'text'|'passsword'|'label']/g;
    var reg2=/['radio'|'checkbox']/g;
    var reg3=/['select']/g;
    if(reg1.test(tagType)){
     res.push(encodeURIComponent(tagName)+"="+encodeURIComponent(tagVal));
    }else if(reg2.test(tagType)&&tag.checked){
     res.push(encodeURIComponent(tagName)+"="+encodeURIComponent(tagVal)); 
    }else if(reg3.test(tagType)){
     for(var j=0;j<tag.options.length;j++){
      if(tag.options[j].selected){
      res.push(encodeURIComponent(tagVal)+"="+encodeURIComponent(tag.options[j].value||tag.options[j].text)); 
      }
     }
    }else{}
   } 
   return res.join(" & ");
  }
   
  // 调用
  var oForm = document.getElementById('target');
  console.log(serializeForm3(oForm));