A hacky implementation of a default prop hoc

default-props-hoc.ts

import * as React from 'react'

type Witness<A, B, C> = (x: A & B) => C

// P = original props
// D = defaulted props
// U = untouched props
function removeProps<P, D extends keyof P, U extends keyof P>(defaults: Pick<P, D>, witness: Witness<Pick<P, D>, Pick<P, U>, P>): (Component: React.ComponentClass<P>) => React.ComponentClass<Pick<P, U>> {
  return Component => class extends React.Component<Pick<P, U>, void> {
    render() {
      return <Component {...this.props} {...defaults} />
    }
  }
}

type Props = {
  name: string,
  surname: string,
  age: number,
  // errorIfDecommented: number
}

class MyComponent extends React.Component<Props, void> {
  render() {
    return <div>hello {this.props.name} {this.props.surname}</div>
  }
}

// the key here seems the function x => x without type annotations which forces type inference
// decommenting the errorIfDecommented prop raises an error because the constraint is violated
const MyDefaultedComponent = removeProps<Props, 'surname', 'name' | 'age'>({ surname: 'Canti' }, x => x)(MyComponent)

export default MyDefaultedComponent