next.js/packages/next/src/shared/lib/is-thenable.ts

/**
 * Check to see if a value is Thenable.
 *
 * @param promise the maybe-thenable value
 * @returns true if the value is thenable
 */
export function isThenable<T = unknown>(
  promise: Promise<T> | T
): promise is Promise<T> {
  return (
    promise !== null &&
    typeof promise === 'object' &&
    'then' in promise &&
    typeof promise.then === 'function'
  )
}