what is the equal sign means in 'T extends Node = Node'

[YeYongFen]

excuse me sir ,I am a rookie，when I red your code as follows. I have no idea what is 'T extends Node = Node' . I Look it up in the document of typescript about generic, but the question about remain unanswered,Can you give me some idea to inspire me

export interface VNode<T extends Node = Node, Props = VNodeProps<Element>> {
  tagName: string | void
  props: Props
  children: Array<VNode> | ReadonlyArray<VNode> | void
  text: string | void
  key: string | number | void
  element: T | void
  namespace: string | void
  scope: string | void

  parent: VNode<Element> | void
}

[TylorS]

Hello @yyf1994gggg! In TypeScript, I believe they are called "type parameter defaults" - see: microsoft/TypeScript#2175

I'll try to explain how this works in practice

// This interface *requires* a type parameter
interface Foo<A> {
  foo: A
}

// This interface provides a *default* when no type parameter is used
interface Bar<A = string> {
  bar: A
}

// always needed
const foo: Foo<number> = {  foo: 1 }
// will error about missing type parameter
const foo: Foo = { foo: 1 }

// When you want to specify something that's not the default
const nonDefaultBar: Bar<number> = { bar: 2 }
// use the default
const defaultBar: Bar = { bar: 'defaults to string' }
// This will error becase Bar<number> is not assignable to Bar
const errorBar: Bar = { bar: 2 } 

I hope this helps you