• worked on this during ES2015 time frame, so never went through stages process
• got punted to later (rightly so!)
• goal for today: resume work on this, reassert committee interest via advancing to stage 1

Intuitions

• sandbox
• iframe without DOM
• principled version of Node's 'vm' module
• sync Worker

Use cases

• security isolation (with synchronous but coarse-grained communication channel)
• plugins (e.g., spreadsheet functions)
• in-browser code editors
• server-side rendering
• testing/mocking (e.g., jsdom)
• in-browser transpilation

Example: simple realm

let realm = new Realm();

let outerGlobal = window;
let innerGlobal = realm.global;

let f = realm.evalScript("(function() { return 17 })");

f() === 17 // true

Reflect.getPrototypeOf(f) === outerFlobal.Function.prototype // false
Reflect.getPrototypeOf(f) === innerGlobal.Function.prototype // true

Example: simple subclass

class EmptyRealm extends Realm {
  constructor(...args) { super(...args); }
  init() { /* do nothing */ }
}

Example: DOM mocking

class FakeWindow extends Realm {
  init() {
    super.init(); // install the standard primordials
    let global = this.global;

    global.document = new FakeDocument(...);
    global.alert = new Proxy(fakeAlert, { ... });
    ...
  }
}

Example: language hooks

class ESNextRealm extends Realm {
  [Realm.directEval](source) { return compile(source); }
  [Realm.indirectEval](source) { ... }
  ...
}

Further considerations

• realm.evalModule()
• module registry
• hooks for manipulating module requests