Could you elaborate in this PR on whatLibraryManagedAttributes is intended to do and give some specific implementations examples?

As background, TS works differently than Flow at the moment with ReactdefaultProps, often causing pain and confusion among junior devs. With this PR,defaultProps will be intuitive. I (think) the ideal behavior is this:

Before

interfaceProps{thing?:string;// thing is optional, but it is always defined because it has a default prop.}classThingextendsReact.Component<Props>{staticdefaultProps={thing:'hello'}render(){console.log(this.props.thing!.length)// forced with !// ...}}// ..<Thing/>// this works

After

interfaceProps{thing?:string;// still optional}classThingextendsReact.Component<Props>{staticdefaultProps={thing:'hello'}render(){console.log(this.props.thing)// inferred because of defaultProps, no ! needed// ...}}// ..<Thing/>// works

@jaredpalmer TheProps type parameter should be the "internal" props (so what you read), while the props returned by the type alias should be the "external" props (the ones you write in a tag). So, instead of

interfaceProps{thing?:string;// still optional}classThingextendsReact.Component<Props>{staticdefaultProps={thing:'hello'}render(){console.log(this.props.thing)// inferred because of defaultProps, no ! needed// ...}}// ..<Thing/>//works

you'd have

interfaceProps{thing:string;// note: not optional within class, but is optional when writing due to the default}classThingextendsReact.Component<Props>{staticdefaultProps={thing:'hello'}render(){console.log(this.props.thing)// strict type from `Props`// ...}}// ..<Thing/>// works, thanks to `defaultProps`

or at least that's what makes sense to me - since theProps generic param is the type of the first argument to the factory/class (which includes all defaults).

got it so it will work like Flow. cool!

This is indeed awesome! In the meantime I wrote an article for React consumers struggling with this "deal breaker" ✌️https://medium.com/@martin_hotell/react-typescript-and-defaultprops-dilemma-ca7f81c661c7

@mhegazy I've synchronized this branch withmaster; can I get this reviewed?

We will need to updatehttps://github.com/Microsoft/TypeScript-Handbook/blob/master/pages/JSX.md

What is the expected behaviour when the props type is a union? Is this expected?

{typeProps={shared:string}&(|{type:'foo';foo:string;}|{type:'bar';bar:string;});classComponentextendsReactComponent<Props>{staticdefaultProps={shared:'yay',};}<Componenttype="foo"/>;// Expected error, got none// Unexpected error// Property 'foo' does not exist on type 'Defaultize<Props, { shared: string; }>'.<Componenttype="foo"foo="1"/>;}

@OliverJAsh this is a different issue i am afraid.

@mhegazy Is it an issue worth tracking? If so, I'll open a separate issue.

yes please.

@OliverJAsh it's probably also worth noting that while this code is intypescript, there won't be any changes until thereact.d.ts is updated to use the new jsx namespace entrypoint; so until that happensdefaultProps still won't do anything.

@mhegazy#25503.

Locking so users open up new issues instead. That way, we can more easily follow up.