require-variance-annotations.md

Enforce variance annotations for all generics where they are supported (detachhead/require-variance-annotations)

Rule Details

this rule enforces that variance annotations (in and out keywords) are used on any generic where thet are allowed.

typescript infers the variance of generics based on their usage, but there are several benefits to defining the variance explicitly:

• it can be a useful for a reader to explicitly see how a type parameter is used at a glance
• can increase performance as the typescript compiler does not need to perform expensive checks to infer the variance where it's explicitly defined
• can prevent unintentional breaking changes in your API if, for example, a new method is added to a class that changes the variance of its generic

Examples of incorrect code for this rule:

type Foo<T> = {
    bar: (value: T) => void
}

interface Bar<T> {
    baz: () => T
}

interface Baz<T> {
    value: T
}

Examples of correct code for this rule:

type Foo<in T> = {
    bar: (value: T) => void
}

interface Bar<out T> {
    baz: () => T
}

interface Baz<in out T> {
    value: T
}

type Qux<T> = T | undefined // Variance annotations are only supported in type aliases for object, function, constructor, and mapped types.

When Not To Use It

if you are using a typescript version older than 4.7

Further Reading

https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#optional-variance-annotations-for-type-parameters