rspec/rules/S4621/javascript/rule.adoc

The TypeScript type system offers a basic support for composite types:

* _Union Types_ represent a value that can be one of the several types. They are constructed using a vertical bar (``++|++``) like the following ``++type NumberOrString = number | string++``.
* _Intersection Types_ combine multiple types into one, so that the object of such type will have all the members of all intersection type elements. They are constructed using an ampersand (``++&++``) like the following ``++type SerializablePerson = Person & Serializable++``. Intersection Types are often used to represent mixins.

Duplicating types when defining a union or interaction type makes the code less readable. Moreover duplicated types might be a simple mistake and another type should be used instead.


== Noncompliant Code Example

[source,javascript]
----
function padLeft(value: string, padding: string | number | string) { // Noncompliant; 'string' type is used twice in a union type declaration
  // ...
}

function extend(p : Person) : Person & Person & Loggable { // Noncompliant; 'Person' is used twice
 // ...
}
----


== Compliant Solution

[source,javascript]
----
function padLeft(value: string, padding: string | number | boolean) {
  // ...
}

function extend(p : Person) : Person & Loggable {
  // ...
}
----


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== Implementation Specification
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