如何使用switch语句查找json对象中是否存在键值对答案

【问题标题】：how to find if key value pair exists in json object using switch statement如何使用switch语句查找json对象中是否存在键值对
【发布时间】：2021-01-12 19:42:11
【问题描述】：

json type :1
var x={
    "address": {
        "county": "abc",
        "state_district": "asd",
        "state": "test",
        "country": "test1",
        "country_code": "test"
    }
 }
Type:2
var x ={ "address": {
            "suburb": "",
            "city_district": "",
            "city": "",
            "county": "",
            "state": "",
            "postcode": "",
            "country": "",
           
        }}
type:3
 var x= {"address": {
            "amenity": "",
            "road": "",
            "town": "",
            "county": "",
            "state_district": "",
            "state": "",
            "postcode": "",
            "country": "",
            "country_code": ""
        }
    }
switch(x.address)
{
    case 'city':
        return x.address.city;
        break;
    case 'village':
        return x.address.village;
        break;    
    default:
        alert('err');}

这是我的 JSON 内容。 address 属性将具有一些额外的键值，具体取决于位置，例如 city、village、suburb 等。

我需要使用 switch 语句检查特定键是否存在。上面显示了带有附加属性的示例 JSON。

【问题讨论】：

标签： javascript json switch-statement

【解决方案1】：

除了Fiouze的回答，你还可以这样做：

// will return true if the property exists.
'city' in x.address

// will return a value if set or undefined if not.
x.address.city

但是，如果您想坚持使用 switch case，您可以通过执行以下操作来遍历属性：

for (const prop in x.address) {
  switch (prop) {
    case 'city': 
      return x.address.city;
    case 'village':
      return x.address.village;
    default:
      alert('err');
  }
}

参考：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in

【讨论】：

【解决方案2】：

您也许可以使用Object.prototype.hasOwnProperty()

    if(x.address.hasOwnProperty('city')){
        return x.address.city
    }else if(x.address.hasOwnProperty('village')){
        return(x.address.village)
    }else{
        alert('err')
    }

See https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Objets_globaux/Object/hasOwnProperty

【讨论】：

【解决方案3】：

Switch 旨在评估单个值，而不是对象的多个属性名称。

以下函数 fn() 复制了您的 switch 块的意图：

const myObject = {
  "address": {
    "amenity": "",
    "road": "",
    "town": "FAKE_TOWN",
    "county": "",
    "state_district": "",
    "state": "",
    "postcode": "",
    "country": "",
    "country_code": ""
  }
};

function fn(obj) {
  const {address} = obj;
  
  // get value of the first property that exists
  // note: `??` is the nullish coalescing operator
  const val =
    address.city ??
    address.village ??
    address.town;

  // duplicate the `default` case behavior
  if (val === undefined) {
    alert('err');
    return undefined;
  }

  return val;
}

console.log(`fn: "${fn(myObject)}"`);

【讨论】：