Null and Undefined

Null and Undefined are data types that handle cases where the value of an expression is not known.

Null

Null is a special data type with only one possible value: null. This value is returned by an expression that does not contain any value.

In the 4D language and for object field attributes, null values are managed through the Null function. This function can be used with the following expressions for setting or comparing the null value:

• object attributes
• collection elements
• variables of the object, collection, pointer, picture, or variant type.

Undefined

Undefined is not actually a data type. It denotes a variable that has not yet been defined. A function (a project method that returns a result) can return an undefined value if, within the method, the function result ($0) is assigned an undefined expression (an expression calculated with at least one undefined variable). A field cannot be undefined (the Undefined command always returns False for a field). A variant variable has undefined as default value.

Null operators

Operation	Syntax	Returns	Expression	Value
Equality	Null = Null	Boolean	a.nullProp = b.nullProp	True
	Null = Undefined	Boolean	a.nullProp = b.undefinedProp	True
	Null = scalar value	Boolean	a.nullProp = 42	False
Inequality	Null # Null	Boolean	a.nullProp # b.nullProp	False
	Null # Undefined	Boolean	a.nullProp # b.undefinedProp	False
	Null # scalar value	Boolean	a.nullProp # 42	True

scalar values are values of type string, Date, Time, Boolean, number, or Blob. When declared, their default value is neither undefined nor null. Other types (Pointer, Picture, Object, Collection) have undefined or null default value. Ex:

var $object : Object
var $text : Text

//$object = null
//$text = "" 

info

Comparisons with Greater than (>), Less than (<), Greater than or equal to (>=), and Less than or equal to (<=) operators are not supported with Null values and return an error.

Undefined operators

Operation	Syntax	Returns	Expression	Value
Equality	Undefined = Undefined	Boolean	a.undefinedProp = b.undefinedProp	True
	Undefined = Null	Boolean	a.undefinedProp = c.nullProp	True
	Undefined = other values	Boolean	a.undefinedProp = 42	False
Inequality	Undefined # Undefined	Boolean	a.undefinedProp # b.undefinedProp	False
	Undefined # Null	Boolean	a.undefinedProp # b.nullProp	False
	Undefined # other values	Boolean	a.undefinedProp # 42	True
Greater than	Undefined > string, Date, Time, Boolean, number	Boolean	a.undefinedProp > "abc"	False
Less than	Undefined < string, Date, Time, Boolean, number	Boolean	a.undefinedProp < "abc"	False
Greater than or equal to	Undefined >= string, Date, Time, Boolean, number	Boolean	a.undefinedProp >= "abc"	False
Less than or equal to	Undefined <= string, Date, Time, Boolean, number	Boolean	a.undefinedProp <= "abc"	False

other values are expressions of any type with a value neither Undefined nor Null.

info

Comparisons of Undefined values with Pointer, Picture, Blob, Object, Collection, Undefined or Null values using Greater than (>), Less than (<), Greater than or equal to (>=), and Less than or equal to (<=) operators are not supported and return an error.

Examples

Here are the different results of the Undefined command as well as the Null command with object properties, depending on the context:

var $vEmp : Object
var $result : Boolean
$vEmp:=New object
$vEmp.name:="Smith"

$vEmp.children:=Null
 
$result:=Undefined($vEmp.name) //False
$result:=($vEmp.name=Null) //False
 
$result:=Undefined($vEmp.children) //False
$result:=($vEmp.children=Null) //True
 
$result:=Undefined($vEmp.parent) //True
$result:=($vEmp.parent=Null) //True

Examples of comparison results with undefined and null values:

var $result : Boolean
var $vObj : Object
var $vVar : Variant

$vObj:=New object
$vObj.null:=Null
//note that $vVar is not assigned 

$result:=($vObj.undefined=42) //False
$result:=($vObj.undefined=$vObj.null) //True
$result:=($vObj.undefined=$vVar)  //True

$result:=($vObj.undefined#$vObj.null) //False
$result:=($vObj.undefined#42) //True
$result:=($vObj.undefined#$vVar) //False

$result:=($vObj.undefined>"hello") //False
$result:=($vObj.undefined>$vVar)  //Error
$result:=($vObj.undefined>$vObj.null)  //Error
$result:=($vVar < 42) //False