javascript - Check if a object is defined, best practice.

Question

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
};

---

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;
};

---

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.

---

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

EDIT:

I don't wan't to use the &&-operator. I am lazy and I'm looking for a reusable method to check if a object (or property of a object) is defined.

:) Thanks!

Answer 1

Use the guard pattern:

if (json.response && json.response.freeOfChargeProduct && json.response.freeOfChargeProduct.details) {
    // you can safely access the price
}  

This is how the guard pattern works.

if (a && a.b && a.b.c) { ... } else { ... }

The first check is "Does the property a exist?". If not, the else-branch gets executed. If yes, then the next check occurs, which is "Does object a contain the property b?". If no, the else-branch executes. If yes, the final check occurs: "Does the object a.b contain the property c?". If no, the else-branch executes. If yes (and only then), the if-branch executes.

Update: Why is it called "guard pattern"?

var value = a && b;  

In this example, the member b (the right operand) is guarded by the && operator. Only if the member a (the left operand) is truthy ("worthy"), only then the member b is returned. If, however, the member a is falsy ("not worthy"), then it itself is returned.

BTW, members are falsy if they return these values: null, undefined, 0, "", false, NaN. Members are truthy in all other cases.