types.ts

/**
 * Exposes type utility functions.
 * @packageDocumentation
 */

/**
 * Does `x` have an undefined property `prop`,
 * or that property is `PropType`?
 *
 * @example
 * ```
 * import { equal } from 'assert'
 * import { haveUndefienedPropertyOrThat } from './types'
 *
 * interface Foo {
 *   bar: number
 * }
 *
 * function isFoo(x: unknown): x is Foo {
 *   return ( // a better predicate.
 *     x instanceof Object &&
 *     typeof (x as { bar?: number }).bar === 'number'
 *   )
 * }
 *
 * equal(haveUndefienedPropertyOrThat(isFoo, {bar: undefined}, 'bar'), true)
 * equal(haveUndefienedPropertyOrThat(isFoo, {bar: 10}, 'bar'), true)
 * equal(haveUndefienedPropertyOrThat(isFoo, {bar: 'bar'}, 'bar'), false)
 * ```
 */
export function haveUndefienedPropertyOrThat<
  PropType,
  Base,
  Prop extends string
>(p: (a: unknown) => a is PropType, x: Base, prop: Prop): boolean {
  const xx = x as {[P in Prop]?: unknown}
  return xx[prop] === undefined || p(x)
}