Find the key existence in JSON object

Question

I have a JSON string

String str = '{ "name":"John", "age":31, "city":"New York" }'

In the above string, city is an optional key and may not be present in the JSON data.

I decoded the json string

var jsonResponse = json.decode(str);

I want to get to know the existence of city key the jsonResponse object.

Answer 1

jsonResponse.containsKey("key");

Answer 2

I am not sure how good is the solution, but it is working.

String str = '{ "name":"John", "age":31, "city":"New York" }';
Map<String, String> jsonResponse = json.decode(str);

if (jsonResponse.containsKey('city')){
   // do your work
}

Instead of capturing the JSON object in var, capture it into a

Map<String, String>
or 
Map<String, Object> //if value is again an object

Answer 3

The result of json.decode is a Map<String, dynamic> with an entry for each key/value pair in the source.

As a Map, you can test for the presence of a key using Map.containsKey. You can also just do a lookup using the "city" key and see if you get null back. In some cases it's more convenient to just use the null value directly than to do an extra check on whether there is something, but that depends entirely on what you are trying to do.

var map = jsonDecode(str);
bool containsCity = map.containsKey("city");
String city = map["city"];
if (city == null) // no city ... 
// Use the null directly if you just replace it with a default.
print("${map["name"]} from ${map["city"] ?? "somewhere"}");
// Use a test if you want to do something different if it's there.
print(map["name"] + (city != null ? " from $city" : "");

The map["city"] == null test differs from map.containsKey("city") in the case where your original source contained the pair "city": null. You need to decide whether that ever happens, and if it does, how you want to treat it. In many cases, it's probably an error, and should be treated as if there was no "city" entry, so I'll usually prefer to use the map["city"] == null test over map.containsKey("city").