8 the realm seam

// DOJO · Module 10 / Exercise 8 — the realm seam
// GOAL: get the HUD's door guard to accept a loot list the mod built,
//       then watch a function the mod wrote run on the host's own
//       array as if no seam were there at all.
// CONCEPT: a custom element runs in the host's one realm. An iframe
//       hands the mod a SECOND realm - its own Array, its own Object,
//       its own copy of every built-in. Values cross the seam intact,
//       but identity checks lie: `instanceof` asks whether the HOST's
//       prototype sits on the value's chain, and a mod-built array
//       inherits from the mod's Array.prototype instead.
// HINT: two questions that sound alike. `x instanceof Array` asks
//       "did MY Array stamp this?". Array.isArray(x) asks "is this an
//       array?" - no chain walk, no realm in the question.
// MIRRORS: the map mod that ships in its own frame. The loot list it
//       hands back is a real array with real items in it, and the HUD
//       still turns the panel away at the door, because the guard is
//       checking whose Array stamped the list rather than what the
//       list is.
// Run: node 08-the-realm-seam.cjs

'use strict';
const assert = require('node:assert');
const vm = require('node:vm');

// The mod is a panel somebody else wrote and shipped. On the HUD it
// would load inside a frame of its own; here node:vm builds the same
// seam in plain Node - a second realm with its own built-ins. Nothing
// below is broken code. The mod is a well-behaved neighbour.
const modRealm = vm.createContext({});
const mod = vm.runInContext(
  `({
    loadout: ['blade', 'tonic', 'map'],
    config: { theme: 'dusk' },
    countSlots(list) {
      return Array.isArray(list) ? list.length : -1;
    },
  })`,
  modRealm,
);

const hostLoadout = ['helm', 'rope'];

// --- TODO 1 --------------------------------------------------------
// Three readings the host takes on values the mod built. Two of them
// use `instanceof`, which walks the value's prototype chain hunting
// for the HOST's Array.prototype or Object.prototype. The third asks
// the value what it is and never looks at a chain.
// In place of null, write the three booleans this object really holds.
const seamReadings = {
  arrayInstanceof: mod.loadout instanceof Array,
  objectInstanceof: mod.config instanceof Object,
  isArray: Array.isArray(mod.loadout),
};

const answerSeam = null; // your answer: the same three keys, booleans

assert.deepStrictEqual(
  seamReadings,
  answerSeam,
  'TODO 1 — what does each reading report across the seam?',
);

// --- TODO 2 --------------------------------------------------------
// acceptsLoadout is the HUD's door guard: the host refuses to draw a
// panel whose loot list is not a list. It waves the host's own arrays
// through and turns the mod's away, which is the whole bug report.
// Rewrite the check so it asks the question the guard actually means,
// and keep saying no to things that are not lists.
function acceptsLoadout(value) {
  return value instanceof Array;
}

assert.strictEqual(
  acceptsLoadout(hostLoadout),
  true,
  'TODO 2 — the guard must still accept the host loot list',
);
assert.strictEqual(
  acceptsLoadout(mod.loadout),
  true,
  'TODO 2 — the guard must accept the mod loot list as well',
);
assert.strictEqual(
  acceptsLoadout('blade'),
  false,
  'TODO 2 — the guard must still reject a value that is not a list',
);

// --- TODO 3 --------------------------------------------------------
// Behaviour crosses the seam even where identity does not. countSlots
// was written inside the mod realm, so over here
// `mod.countSlots instanceof Function` is false while
// `typeof mod.countSlots` is still 'function' - and the host is about
// to call it on an array the HOST built. Read countSlots above, decide
// what it sees from its side of the seam, and write the number it
// hands back for hostLoadout.
const slots = mod.countSlots(hostLoadout);

const answerSlots = null; // your answer

assert.strictEqual(
  slots,
  answerSlots,
  'TODO 3 — what does the mod function report for the host list?',
);

console.log('PASS');