检查对象是否已定义，最佳做法。 [英] Check if a object is defined, best practice.

问题描述

我从ajax请求获得以下JSON响应。

I have the following JSON response from a ajax-request.

var json = {
    "response": {
        "freeOfChargeProduct": {  
        "description": "Product",  
        "orderQty": 5,
        "productName": "XYZ",
        "qty": 6,
        "details": {
            "price": 55.5, 
            "instock": "true",
            "focQuantity": 1
        }
    }, 
    "orderLineId": 4788,
    "totalOrderLinePrice": "741.36",
    "totalOrderPrice": "1,314.92",
    "totalQty": 17
};

---

JSON并不总是返回freeOfChargeProduct 财产。因此，如果我想获得freeOfChargeProduct价格，那么我必须执行以下操作：

---

The JSON dosen't always return a "freeOfChargeProduct" property. So if I want to get the "freeOfChargeProduct" price, then I have to do the following:

var getFreeOfChargeProductPrice = function() { 
   var r = json.response;
   if (r && r.freeOfChargeProduct && r.freeOfChargeProduct.details) {
      return r.freeOfChargeProduct.details.price;         
   }
   return null;
};

---

没问题。但检查对象中的每个属性都非常烦人，因此我创建了一个函数来检查对象中的属性是否已定义。

---

No problems. But it's very annoying to check every property in the object, so I created a function that check if a property in a object is defined.

var getValue = function (str, context) {
    var scope = context || window,
        properties = str.split('.'), i;
    for(i = 0; i < properties.length; i++) {
      if (!scope[properties[i]]) {                       
         return null;
      } 
      scope = scope[properties[i]];        
    }
    return scope;
};

---

---

var price = getValue('json.response.freeOfChargeProduct.details.price');
// Price is null if no such object exists.

---

现在我的问题：这是好还是检查对象中是否存在属性的错误方法？还有更好的建议/方法吗？

---

Now to my question: Is this a good or bad way to check if a property exists in an object? Any better suggestions/methods?

编辑：

我不喜欢不要使用&& -operator。我很懒，而且我正在寻找一种可重用的方法来检查对象（或对象的属性）是否被定义。

:)谢谢！

推荐答案

if(x && typeof x.y != 'undefined') {
    ...
}

// or better
function isDefined(x) {
    var undefined;
    return x !== undefined;
}

if(x && isDefined(x.y)) {
    ...
}

这适用于JavaScript中的任何数据类型，甚至是零的数字。如果您要检查对象或字符串，只需使用 x&&在if语句中xy ，或者如果你已经知道x是一个对象， if（xy）...

This will work for any data type in JavaScript, even a number that is zero. If you are checking for an object or string, just use x && x.y within the if statement, or if you already know that x is an object, if(x.y) ...

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