I've seen this pattern and I don't understand what is going on.

interface Foo<T extends string | string[] = string | string[]> { }

I have an interface with a generic that extends three items. The second item contains an assignment where a string array is assigned to a string?

I haven't been able to find an answer on the forums because I find this problem hard to articulate.

Any help would be appreciated thanks.

CodePudding user response：

The expression is confusing, and it is not a Union of 3 items.

In fact it just says T extends string | string[], and the default type is string | string[] if T is not specified.

So const foo: Foo will be infer to const foo: Foo<string | string[]>

Here are some transformations to help you to understand what's happening.

1:

interface Foo<T extends (string | string[]) = (string | string[])> { }

2:

type StringOrArray = string | string[]
interface Foo<T extends StringOrArray = StringOrArray > { }