Type Guard in jsdoc comment

type-guard-example.js

/** @typedef {{swim: () => void}} Fish */
/** @typedef {{fly: () => void}} Bird */

/**
 * @param {Fish | Bird} pet
 * @returns {pet is Fish}
 */
function isFish(pet) {
  return "swim" in pet;
}

/** @type {Fish | Bird} */
let pet = getPet();

// at this point "pet" is either a Fish or Bird

if (isFish(pet)) {
  // at this point you (and tsc and intellisense) know you have a Fish
  pet.swim();
} else {
  // at this point you (and tsc and intellisense) know you have a Bird
  pet.fly();
}