Add containsKey

API-Reference.md

@@ -36,6 +36,7 @@ Some parts have been simplified to be easier to understand, so if you look at th
     - [JsonObject::begin\(\) / JsonObject::end\(\)](#jsonobjectbegin--jsonobjectend)
     - [JsonObject::createNestedArray\(\)](#jsonobjectcreatenestedarray)
     - [JsonObject::createNestedObject\(\)](#jsonobjectcreatenestedobject)
+    - [JsonObject::containsKey\(\)](#jsonobjectcontainskey)
     - [JsonObject::get\(\)](#jsonobjectget)
     - [JsonObject::measureLength\(\)](#jsonobjectmeasurelength)
     - [JsonObject::measurePrettyLength\(\)](#jsonobjectmeasureprettylength)
@@ -1000,6 +1001,39 @@ will print
 ```
 
 
+### JsonObject::containsKey()
+
+##### Description
+Tests if a key exists in the `JsonObject`.
+
+##### Signature
+
+```c++
+bool containsKey(const char* key) const;
+bool containsKey(const String& key) const;
+bool containsKey(const std::string& key) const;
+```
+
+##### Arguments
+
+`key`: the key to test.
+
+##### Return value
+
+`true` if the key is present is the `JsonObject`; `false` if not.
+
+##### Example
+
+```c++
+StaticJsonBuffer<256> jsonBuffer;
+JsonObject& root = jsonBuffer.createObject();
+root["city"] = "Paris";
+
+bool hasCity = root.containsKey("city"); // true
+bool hasCountry = root.containsKey("country"); // false
+```
+
+
 ### JsonObject::get()
 
 ##### Description