== Why is this an issue?

Union types represent a value that can be one of the several types. When a union type is used for a function parameter and it is accepting too many types, it may indicate the function is having too many responsibilities. Sometimes it's worth creating a type alias for this union type. In all cases, the code should be reviewed and refactored to make it more maintainable.


=== Noncompliant code example

With the default threshold of 3:

[source,javascript]
----
let x: MyType1 | MyType2 | MyType3 | MyType4; // Noncompliant

function foo(p1: string, p2: MyType1 | MyType2 | MyType3 | MyType4) { // Noncompliant
    // ...
}
----


=== Compliant solution

[source,javascript]
----
type MyUnionType = MyType1 | MyType2 | MyType3 | MyType4; // Compliant, "type" statements are ignored
let x: MyUnionType;

function foo(value: string, padding: MyUnionType) {
    // ...
}
----


=== Exceptions

This rule ignores union types part of ``++type++`` statement:

[source,javascript]
----
type MyUnionType = MyType1 | MyType2 | MyType3 | MyType4;
----

It also ignores union types used with TypeScript utility types:

[source,javascript]
----
type PickedType = Pick<SomeType, 'foo' | 'bar' | 'baz' | 'qux'>;
----

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Refactor this union type to have less than X elements.


=== Parameters

.max
****

----
3
----

Maximum elements authorized in a union type definition.
****


=== Highlighting

All the elements of the union type


endif::env-github,rspecator-view[]

== Resources
=== Documentation

* https://www.typescriptlang.org/docs/handbook/utility-types.html[TypeScript Utility Types]