Accessing Child Elements Of JSON Objects And Arrays

JSON Objects

We can access child elements of a TJSONValue object by using square brackets syntax with a path:

Object[‘path.to.a.value’]

Or a sequence of square brackets with paths, because each [] call simply returns another TJSONValue:

Object[‘path’][‘to’][‘a’][‘value’]

Dots in the path allow to get access to child elements. For arrays square brackets with indices can be used:

Object[‘subobject.subarray[2].key’]

The result of the square brackets operator is a TJSONValue object, so it must be tested for a specific type before accessing its values.

The result of the square brackets operator is nil, if the specified path does not exist in the object.

var
    obj: TJSONObject;
begin
    obj := TJSONValue.Parse(‘{“boolValue”: true}’) as TJSONObject;
 if obj[‘boolValue’].IsBoolean then
 begin
 if obj[‘boolValue’].AsBoolean then
            LogInfo(‘The boolean value is true’)
 else
            LogInfo(‘The boolean value is false);
 end;
    obj.Free;
end;

The following list of TJSONValue methods can be used to cast its value into one of the types:

• AsString – returns a standard string value (not a TJSONString object)
• AsInt – returns a standard integer value (not a TJSONNumber object)
• AsInt64 – returns a standard int64 value (not a TJSONNumber object)
• AsNumber – returns a standard double value (not a TJSONNumber object)
• AsBoolean – returns a standard boolean value (not a TJSONBool object)
• AsObject – returns a TJSONObject object (we used it for downcasting in our samples)
• AsArray – returns a TJSONArray object

These methods may throw an error if the casting or conversion can not be done due to a type mismatch. If you are not sure what the type of the value is, use one of the methods described in “JSON Variable Types” first.

JSON Arrays

If you have a TJSONArray object, you can access its elements by an integer index using square brackets syntax:

LogInfo(Array[0].AsString);

Remember: Elements of a JSON array are zero indexed. So they can be accessed by the following indices:

[0 … Count – 1]

You can also obtain a number of children of a JSON array using Count method and iterate through the elements as in the example:

var
    arr: TJSONArray;
    i: integer;
begin
   arr := TJSONValue.Parse('["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]') as TJSONArray;
 for i := 0 to arr.Count - 1 do
      LogInfo(arr[i].AsString);
   arr.Free;
end;

Output

Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday