如何在for循环外部获取并使用循环内筛选出的Rice对象数组？

我是JavaScript新手，若术语使用有误请见谅。我在for循环内创建数组，尝试筛选productname为Rice的数据，但在循环外打印时仅显示一行内容。希望能在for循环外部查看所有筛选出的Rice相关对象数组，代码如下：

const datastructure= [
    {"months":"January","productname":"Rice","production":"10","hector":"2"},
    {"months":"February","productname":"Rice","production":"11","hector":"2"},
    {"months":"March","productname":"Rice","production":"12","hector":"2"},
    {"months":"April","productname":"Rice","production":"13","hector":"2"},
    {"months":"January","productname":"Fruit","production":"14","hector":"2"},
    {"months":"February","productname":"Fruit","production":"15","hector":"2"},
    {"months":"Markch","productname":"Fruit","production":"16","hector":"2"},
    {"months":"April","productname":"Fruit","production":"16","hector":"2"},
];
for(i in datastructure){
  if(datastructure[i].productname=="Rice")
  {
  var months=datastructure[i].months
  var  productname=datastructure[i].productname
  var  production=datastructure[i].production
  var  hector=datastructure[i].hector
formatedata =[({'months':months, 'productname':productname,'production':production,'hector':hector})];
 console.log(formatedata)
  }
}

问题根源 #

每次循环匹配到Rice数据时，你都用formatedata = [新对象]重新赋值了数组，而不是往数组里添加元素。这导致循环结束后，formatedata只保留了最后一次循环创建的单元素数组，所以外部打印只会显示一行。

修复后的代码 #

方法1：修正原循环逻辑

先在循环外初始化一个空数组，每次匹配到数据时用push()方法添加元素：

const datastructure= [
    {"months":"January","productname":"Rice","production":"10","hector":"2"},
    {"months":"February","productname":"Rice","production":"11","hector":"2"},
    {"months":"March","productname":"Rice","production":"12","hector":"2"},
    {"months":"April","productname":"Rice","production":"13","hector":"2"},
    {"months":"January","productname":"Fruit","production":"14","hector":"2"},
    {"months":"February","productname":"Fruit","production":"15","hector":"2"},
    {"months":"Markch","productname":"Fruit","production":"16","hector":"2"},
    {"months":"April","productname":"Fruit","production":"16","hector":"2"},
];
// 循环外初始化空数组
const formatedata = [];
for(let i in datastructure){
  if(datastructure[i].productname === "Rice"){
    const { months, productname, production, hector } = datastructure[i];
    // 用push添加元素，而不是重新赋值
    formatedata.push({ months, productname, production, hector });
  }
}
// 循环外打印所有结果
console.log(formatedata);

方法2：用Array.filter()更简洁（推荐）

JS数组内置的filter()方法专门用来筛选元素，代码更简洁易读：

const datastructure= [
    {"months":"January","productname":"Rice","production":"10","hector":"2"},
    {"months":"February","productname":"Rice","production":"11","hector":"2"},
    {"months":"March","productname":"Rice","production":"12","hector":"2"},
    {"months":"April","productname":"Rice","production":"13","hector":"2"},
    {"months":"January","productname":"Fruit","production":"14","hector":"2"},
    {"months":"February","productname":"Fruit","production":"15","hector":"2"},
    {"months":"Markch","productname":"Fruit","production":"16","hector":"2"},
    {"months":"April","productname":"Fruit","production":"16","hector":"2"},
];
// 直接筛选出productname为Rice的所有对象
const formatedata = datastructure.filter(item => item.productname === "Rice");
console.log(formatedata);

额外提示 #

• 尽量用let或const声明变量，避免使用var（var存在变量提升和作用域问题）；
• 遍历数组优先用for...of或者数组方法（如filter、map），for...in更适合遍历对象属性；
• 可以用对象解构简化代码，比如const { months, productname } = datastructure[i]，不用逐个赋值。

内容的提问来源于stack exchange，提问作者azowad