Variant

Variant stores any Smart Pascal base or complex type, JavaScript object or array.

You can use variant to store object values, since you do not have to predefine the structure by hand. In Pas2JS these kind of structures can be achieved automatically using "inline assembler" by grace of targeting Javascript directly.

Variants holding a JavaScript object cannot be directly casted from/to TObject. One of the reason for this is the obfuscation but even with obfuscation disabled, Pas2JS compiler has to rename inherited methods, which are not supported in the JavaScript. Similar scoping issues happen in variety of other situations (nested procedures f.i.).

Variant is supported as a data-type, but can only be used as containers. The compiler has no operators that deals with them (so you cannot write: A := 1 + myVariant, because there is no way to predict the resulting type without considerable overhead and loss of performance).

When interacting with ordinary javascript libraries or a remote webservice you natually have to go native with your data structures.

Under Pas2JS this can be handled in a variety of ways but the two most common approaches are to either use records or variants.

Records in Pas2JS work just like they do in other object pascal environments. And since Smart compiles to javascript rather than machine opcodes, a record will always compile to an ordinary javascript structure. You should note that javascript really does not distinguish between what we would call a record and objects. In javascript everything is an object.

This means that when you want to interact with native javascript libraries that expects a structure of values, you can use records to match it directly.

Published record properties can be automatically assigned to a field of a variant object (and not directly to the variant object itself).

type

TRec = record

public

A: integer;

published

B: integer;

S: string;

end;

procedure TForm1.W3Button7Click(Sender: TObject);

var

r: TRec;

v: Variant;

s: string;

begin

r.A := 17;

r.B := 42;

r.S := 'Smart!';

v := TVariant.CreateObject;

v.field := r;

asm @s = JSON.stringify(@v); end;

Writeln(s);

end;

You can use TVariant object to create javascript object structure exactly the same as that produced by the record.

Under Pas2JS, variants are used to transport information between raw JavaScript methods. You can however have variant parameters and function results, but these can only be accessed by methods using ASM section.

The resulting javascript structure will be exactly the same as that produced by the record. One difference to keep in mind though, is that the compiler will output the names "as is". There is no way for Smart to map the content of a native object so make sure you get the names right.

Field names can be specified as strings to provide for naming conventions, valid in JavaScript but not in Object Pascal.

Creating reader/writer class for storage is very easy. Once you have populated your object's data, you can use JSON to serialize and make it portable. Take a look at this example:

To force a variant cast explicitly, you can use TVariant object that contains basic variant functions.

TVariant = class

public

class function AsInteger(const aValue: Variant): Integer; static;

class function AsString(const aValue: Variant): String; static;

class function AsFloat(const aValue: Variant): Float; static;

class function AsObject(const aValue: Variant): TObject; static;

class function AsBool(const aValue: Variant): Boolean; static;

class function IsNull(const aValue: Variant): Boolean; static;

class function IsString(const aValue: Variant): Boolean; static;

class function IsNumber(const aValue: Variant): Boolean; static;

class function IsInteger(const aValue: Variant): Boolean; static;

class function IsBool(const aValue: Variant): Boolean; static;

class function IsNAN(const aValue: Variant): Boolean; static;

class function Properties(const aValue: Variant): TStrArray; static;

class function OwnProperties(const aValue: Variant): TStrArray; static;

class function CreateObject: Variant; static;

class function CreateArray: Variant; static;

end;

// Variants support functions:

VarIsValidRef(const aRef: Variant): Boolean;

ObjToVariant(value: TObject): Variant;

VariantToObj(const Value: Variant): TObject;

VariantExtend(const target, prop: Variant) : Variant; overload;

VariantExtend(target: JObject; const prop: Variant): JObject; overload;

VarIsClear(v: Variant) : Boolean;

VarIsEmpty(v: Variant) : Boolean;

VarIsNull(v: Variant) : Boolean;

VarIsValidRef function: Indicates whether the specified variant is neither Null nor undefined.

VarIsNull function: It returns true if the given variant contains the value Null. If the variant contains any other value, the function result is false.

Note: Do not confuse a Null variant with an unassigned/undefined variant. A Null variant is still assigned, but has the value Null. Unlike unassigned variants, Null variants can be used in expressions and can be converted to other types of variants.

VarIsEmpty function: It returns true if the given variant contains the value Unassigned. If the variant contains any other value, the function result is false.

Note: Do not confuse an unassigned/undefined variant with a Null variant. A Null variant is still assigned, but has the value Null. Unlike unassigned variants, Null variants can be used in expressions and can be converted to other types of variants.

VarIsClear function: It returns true if the given variant’s value is undefined. The value can be undefined for any of several reasons:

- The Variant’s value may be an interface type that has been set to nil (Delphi) or NULL (C++).

- The Variant may be a custom variant that returns true from its IsClear method.

Note: Do not confuse an unassigned variant with a Null variant. A Null variant is still assigned, but has the value Null. Unlike unassigned variants, Null variants can be used in expressions and can be converted to other types of variants.