Is the data a null value ?
Source position: fpjson.pp line 206
|  public property TJSONData.IsNull : Boolean | 
IsNull is True only for JSONType=jtNull, i.e. for a TJSONNull instance. In all other cases, it is False. This value cannot be set.
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 | Possible types of JSON data. | |
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 | The native JSON data type represented by this object. | |
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 | Class to represent boolean JSON data. | |
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 | Access the raw JSON value as a float. | |
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 | The value of this data object as a variant. | |
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 | Access the raw JSON value as an 64-bit integer. | |
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 | Access the raw JSON value as a string. | |
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 | Access the raw JSON value as an 32-bit integer. | |
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 | Access the raw JSON value as a boolean. | |
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 | Return a JSON representation of the value. |