TJSONValueType

TJSONValueType = (jsString, jsNumber, jsBoolean, jsArray, jsObject, jsNull)

Example

procedure ReadJSONValue(Item: TJSONValue; var Value: Variant);
begin
  if Item.ValueType = jsNull then
    Value := Null
  else if Item.ValueType = jsString then
    Value := Item.AsString
  else
    Value := Item.AsInfo;
end;

Usage

TJSONValueType reports the JSON value class as string, number, Boolean, array, object or null.

Members

Value	Ordinal	Meaning
jsString	0	A TJSONString, with separate mutable null state and incomplete escape handling.
jsNumber	1	A TJSONInteger or TJSONDouble, with exponent/locale limitations.
jsBoolean	2	A TJSONBoolean.
jsArray	3	An owned ordered TJSONArray.
jsObject	4	An owned named-value TJSONObject, including lookup/serialization quirks.
jsNull	5	A TJSONNull that always reports null.

Behavior and boundaries

• ValueType is class-derived and read-only. Assigning IsNull := True on an ordinary value does not change its ValueType to jsNull; test both when null-state provenance matters.
• AsArray and AsObject use Velox checked class casts and raise for the wrong kind.
• Several scalar accessors accept more than one kind by converting AsString; locale, format, overflow and invalid-text errors can still occur.
• AsBoolean returns false for null; non-Boolean values compare their text case-insensitively with true. This can conceal malformed Boolean input unless ValueType is checked first.
• Objects and arrays require traversal; do not serialize or compare them as if AsString were a normalized scalar representation.

Parser and serializer limits

• Literal true, false and null input is accepted case-insensitively, outside RFC 8259's lowercase grammar.
• \u escapes are not decoded: the u and four hexadecimal digits all remain text.
• Exponent-only numbers are misclassified as integers; double parsing/formatting uses host-locale StrToFloat/FloatToStr.
• Empty-object compact AsString returns } and property names are not JSON-escaped. Other uncommon control characters can also be emitted unescaped.
• TJSONObject.Values[missing] adds a named null, so a lookup can mutate the object. Lookup is case-insensitive first-match and duplicate names are permitted.
• Scalar Clear marks null without erasing stored data; arrays/objects always report non-null.

Additional Technical Info

TJSONValueType is assigned by the base TJSONValue constructor from the concrete runtime class. It identifies JSON syntax kind, allowing scripts to select a compatible accessor before conversion.

The constructor recognizes array, string, Boolean, null, number and object subclasses. An unknown direct subclass raises EJSONError rather than receiving a fallback enum value.

Related Code Library entries

• TJSONValue.ValueType - exact constructor-derived property.
• TJSONValue.IsNull - separate mutable null flag.
• TJSONValue.AsArray - checked array cast.
• TJSONValue.AsObject - checked object cast.

External references

• RFC 8259 JSON values - standard JSON value kinds.
• Embarcadero enumerated types - Delphi ordinal semantics.
• Free Pascal ordinal types - compatible enumeration concepts.

Created 2026-07-15