ECMAScript Proposal, specs, and reference implementation for Realms
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Latest commit bb854a8 Oct 9, 2018

ECMAScript spec proposal for Realms API


Current Stage

This proposal is at stage 2 of the TC39 Process.


  • @dherman
  • @caridy
  • @erights

Spec Text

You can view the spec rendered as HTML.


Build Status Coverage Status dependency status dev dependency status License

A shim implementation of the Realm API can be found under the shim folder.

You can play around with the current version of the shim with the Realm playground.



  • worked on this during ES2015 time frame, so never went through stages process
  • got punted to later (rightly so!)
  • goal of this proposal: resume work on this, reassert committee interest via advancing to stage 2
  • original idea from @dherman: What are Realms?


  • 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 g = window; // outer global
let r = new Realm(); // root realm

let f = r.evaluate("(function() { return 17 })");

f() === 17 // true

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

Example: simple compartment

let g = window; // outer global
let r1 = new Realm(); // root realm
let r2 = new{ intrinsics: "inherit" }); // realm compartment

let f = r1.evaluate("(function() { return 17 })");

f() === 17 // true

Reflect.getPrototypeOf(f) === g.Function.prototype // false
Reflect.getPrototypeOf(f) === // true
Reflect.getPrototypeOf(f) === // 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 =;

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

Example: parameterized evaluator

Transform Trap

The transform trap provides a way to preprocess any sourceText value before it is evaluated, and it applies to direct and indirect evaluation alike. E.g.:

const r = new Realm({
  transform(sourceText) {
    return remapXToY(sourceText);
}); = 1;
const a = r.evaluate(`let x = 2; eval("x")`);      // yields 1 after remapping `x` to the global `y`.
const b = r.evaluate(`let x = 3; (0, eval)("x")`); // yields 1 after remapping `x` to the global `y`.

For mode details about how to implement a JS dialects with Realms, check the following gist:

Example: controlling direct evaluation

The isDirectEval trap provides a way to control when certain invocation to an eval identifier qualifies as direct eval. This is important if you plan to replace the eval intrinsic to provide your own evaluation mechanism:

const r = new Realm({
  isDirectEval(func) {
    return func === r.customEval;
function customEval(sourceText) {
  return compile(sourceText);
} = customEval; // providing a custom evaluation mechanism
const source = `
  let x = 1;
  (function foo() {
    let x = 2;
    eval('x');      // yields 2 if `compile` doesn't alter the code
    (0,eval)('x');  // yields 1 if `compile` doesn't alter the code

Import Trap

The import trap has been removed for stage 2. We might bring it back at some point.


Updating the spec text for this proposal

The source for the spec text is located in spec/index.emu and it is written in ecmarkup language.

When modifying the spec text, you should be able to build the HTML version in index.html by using the following command:

npm install
npm run build
open index.html

Alternative, you can use npm run watch.