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Modify rule S4782: Adapt to LaYC (#2681)

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Co-authored-by: Yassin Kammoun <52890329+yassin-kammoun-sonarsource@users.noreply.github.com>
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rules/S4782/javascript/rule.adoc
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@ -1,56 +1,58 @@
== Why is this an issue?
In TypeScript there are several ways to declare a property with ``++undefined++`` value: adding ``++| undefined++`` in the property type or using optional property syntax (``++?++`` after its name). Use ``++| undefined++`` syntax when you want to be explicit that an object has that property, in that case TypeScript compiler will not allow omitting it:
In TypeScript, there are two ways to define properties or parameters that are potentially ``++undefined++``:
----
interface Person {
name: string;
address: string | undefined;
}
let John = { name: "John" }; // will not compile
let John = { name: "John", address: undefined }; // will compile, we want to be explicit when person does not have home
----
Use optional property syntax for properties holding some additional information.
----
interface Person {
name: string;
pet?: string;
}
let John = { name: "John" }; // will compile
let John = { name: "John", pet: undefined }; // will compile, there is no pet like for the object on previous line
let John = { name: "John", pet: "Benji" }; // will compile
----
Using ``++| undefined++`` for optional property is redundant, it can be omitted without change to the actual type. Still if you want to force the property in the object consider using only ``++| undefined++`` without ``++?++``.
=== Noncompliant code example
[source,javascript]
----
interface Person {
name: string;
address? : string | undefined; // Noncompliant, "?" should be removed
pet?: Animal | undefined; // Noncompliant, "undefined" should be removed
}
----
=== Compliant solution
* Union with `undefined`: Adding ``++| undefined++`` in the property type makes the property __required__, but can be `undefined`. Use this syntax when you want to be explicit that an object should provide that property, in which case the TypeScript compiler will not allow omitting it.
[source,javascript]
----
interface Person {
name: string;
address: string | undefined;
pet?: Animal;
}
let John = { name: "John", address: undefined };
----
* Optional property syntax (``++?++`` after its name): The property is __optional__, which means that an object can omit it and let the TypeScript compiler provide it as being `undefined`.
[source,javascript]
----
interface Person {
name: string;
address?: string;
}
let John = { name: "John" };
----
This rule checks for optional property declarations that use both the `?` syntax and unions with `undefined`.
[source,javascript,diff-id=1,diff-type=noncompliant]
----
interface Person {
name: string;
address?: string | undefined; // Noncompliant: using both syntaxes is redundant
}
----
Choose one of the syntaxes to declare optional properties and remove the other one. Consider using only ``++| undefined++`` if you want to make the property explicit in the object.
[source,javascript,diff-id=1,diff-type=compliant]
----
interface Person {
name: string;
address?: string;
}
----
== Resources
=== Documentation
* https://www.typescriptlang.org/docs/handbook/2/objects.html#optional-properties[TypeScript - Optional Properties]
* https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#union-types[TypeScript - Union Types]
ifdef::env-github,rspecator-view[]
'''