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:

let x: MyType1 | MyType2 | MyType3 | MyType4; // Noncompliant

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

Compliant solution

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:

type MyUnionType = MyType1 | MyType2 | MyType3 | MyType4;

It also ignores union types used with TypeScript utility types:

type PickedType = Pick<SomeType, 'foo' | 'bar' | 'baz' | 'qux'>;

Resources

Documentation