var kvArray = [{key: 1, value: 10}, 
               {key: 2, value: 20}, 
               {key: 3, value: 30}];
var reformattedArray = kvArray.map(function(obj,index) { 
  console.log(index)
   var rObj = {};
   rObj.id=index;//新增id屬性
   rObj[obj.key] = obj.value;//修改屬性
   return rObj;
});
console.log(reformattedArray);

var reformattedArray2 = kvArray.map(function(obj,index) { 
   obj.id=index;//新增id屬性
   return obj;//如果不返回則輸出： Array [undefined, undefined, undefined]
});
console.log(reformattedArray2);
 

輸出：

> 0

> 1

> 2

> Array [Object { 1: 10, id: 0 }, Object { 2: 20, id: 1 }, Object { 3: 30, id: 2 }]

> Array [Object { key: 1, value: 10, id: 0 }, Object { key: 2, value: 20, id: 1 }, Object { key: 3, value: 30, id: 2 }]

參考：

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array/map