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Tech Tip: User This Keyword to access Object properties in Object Formula
PRODUCT: 4D | VERSION: 17 R3 | PLATFORM: Mac & Win
Published On: February 28, 2019
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One of new features in V17 R3 is the addition of new Command New Formula. It creates an object formula based on the formula expression. It also allows adding a formula as property of object and can be invoked using object notation sytax. For example:

C_OBJECT($f)
$f:=New formula(1+2)
$o:=New object("myFormula";$f)
$o.myFormula() //returns 3

Besides quick and easy access, assigning formula as property to an object has another advantage: it allows access of the entire object within the formula’s context using the keyword This. For example:

$o:=New object("Name";"John";"Age";20)
$o.myFomular:=New formula(This[$1]:=$2)
// {"Name":"John","Age":20,"myFomular":"[object Formula]"}


$o.myFomular("Age";23) // {"Name":"John","Age":23,"myFomular":"[object Formula]"}
$o.myFomular("Name";"Jack") // {"Name":"Jack","Age":23,"myFomular":"[object Formula]"}

The key word in the formula refers to $o object and has full access to every property. It enables the formula to work with the data of its outer object. Below is an example to add a formula to calculate birthyear based on the age of a person:

$person:=New object("Name";"John";"Age";20)
$person.getBirthYear:=New formula(Year of(Current date) -This.Age)
$person.getBirthYear()
//result: 1999
$person.Age:=30
$person.getBirthYear()
//result: 1989

The ability to execute formula on an object offers a clean way to organize code and access its properties offers a clean way to calculate result based on object data dynamically.