typed react – props as `type` or an `interface`

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It is 2020 now and I would favor type in almost all cases with React props (general type vs interface post is here). Common cases that can be only expressed with type aliases:

// given some props from another comp that are to be altered
type ExternalProps = { a: string; b: { c: number } };

type Props_IndexType = ExternalProps["b"]; // { c: number; }
type Props_MappedType = { [K in keyof ExternalProps]: number }; // { a: number; b: number; }
type Props_DiscriminatedUnionType = { tag: "tag1"; foo: string } | { tag: "tag2"; foo: boolean}
type Props_typeOf = { foo: string } & typeof defaultProps; // see class comp example

// conditional types - ok, this one is a bit contrived, but you get the point
type Props_ConditionalType<T> = T extends true ? { a: string } : { b: number };
const Comp = <T extends {}>(props: Props_ConditionalType<T>) =>
  <div>{"a" in props && (props as any).a}</div>
render(<Comp<true> a="foo" />, document.getElementById("root"));

Class component example for illustration (OP mentions them, but above cases also apply for Hooks):

// cannot do that with interfaces
type Props = ({ tag: "tag1"; foo: string } | { tag: "tag2"; foo: boolean }) &
  typeof defaultProps;
type State = typeof initState;

const defaultProps = { a: "A" };
const initState = { c: "C" };

class App extends React.Component<Props, State> {
  static readonly defaultProps = defaultProps;
  state = initState;

  render() { ... }
}

render(<App tag="tag1" foo="foo" />, document.getElementById("root"));

The only cases, I would consider interfaces:

  • You do declaration merging of prop types in the global scope (uncommon nowadays)
  • You want to hide type implementation details, as interfaces create a new name used in error messaged, IDE type infos etc. (docs)

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