Future Javascript: ShadowRealms
Tags: javascript, tutorial, webdev, programming
Published: 2022-03-07 10:21:56

It sounds dark and mysterious - but it's just another future Javascript feature. The `ShadowRealm` is a new feature coming to Javascript, which will let us create a separate global context from which to execute Javascript. In this article, we'll look at what the ShadowRealm is, and how it works.

## Support for ShadowRealms in Javascript
`ShadowRealm`s are a Javascript proposal, currently at Stage 3. As such, `ShadowRealm`s do not have support in browsers or natively in server side languages like Node.JS, and given it has had many changes over the years, there is no stable babel or `npm` plugin to polyfill the functionality. However, given it has reached Stage 3, this means there won't be very many changes going forward, and we can expect ShadowRealms to have native support at some point in the future.

## How ShadowRealms in Javascript work
A `ShadowRealm` is ultimately a way to set up a totally new environment with a different global object, separating the code off from other realms. When we talk about a global object in Javascript, we are referring to the concept of `window` or `globalThis`. The problem that `ShadowRealm` ultimately tries to solve, is to reduce conflict between different sets of code, and provide a safe environment for executing and running code that needs to be run in isolation. It means less pollution in the global object from other pieces of code or packages. As such, code within a ShadowRealm cannot interact with objects in different realms.

### ShadowRealm Use Cases:
1. Code editors where the user can edit code, and which we don't want to interact with the main webpage.
2. Plugins that can be executed independently.
3. Emulating the DOM in a separated environment, i.e. if we need to know the scroll position in certain scenarios, we could emulate it within a ShadowRealm so that the user scrolling on the main webpage would not affect the `window.top` variable in our emulation.

ShadowRealms run on the same thread as all other Javascript - so if you want to multi-thread your Javascript, you still have to use Web Workers. As such, a ShadowRealm can exist within a worker, as well as within a regular Javascript file. ShadowRealms can even exist within other ShadowRealms.

## Creating a ShadowRealm in Javascript
Let's look at how a ShadowRealm actually looks in code. The first thing we have to do is call a new ShadowRealm instance. We can then import some Javascript into our Realm, which will run within it. For this, we use a function called importValue, which works effectively in the same way as import.

```javascript
let myRealm = new ShadowRealm();

let myFunction = await myRealm.importValue('./function-script.js', 'analyseFiles');

// Now we can run our function within our ShadowRealm
let fileAnalysis = myFunctions();
```

In the above example, analyseFiles is the export name we are importing from `function-script.js`. We then capture and store this export within myFunction. Significantly, the export we import into our realm must be callable, so it must effectively be a function we can run.

Our `function-script.js` file is just a normal Javascript file with an export. It may look something like this:

```javascript
export function analyseFiles() {
    console.log('hello');
}
```

The ShadowRealm is totally separate from other global objects we may have, such as window or globalThis.

Similar to other imports, we can use the curly bracket import notation:

```javascript
let myRealm = new ShadowRealm();

const { runFunction, testFunction, createFunction } = await myRealm.importValue('./function-script.js');

let fileAnalysis = runFunction();
```
Or, we can create multiple promises that all translate into an array if we want to use named importValues.

```javascript
let myRealm = new ShadowRealm();

const [ runFunction, testFunction, createFunction ] = await Promise.all([
    myRealm.importValue('./file-one.js', 'runFunction'),
    myRealm.importValue('./file-two.js', 'testFunction'),
    myRealm.importValue('./file-three.js', 'createFunction'),
]);

let fileAnalysis = runFunction();
```

## Executing Code with evaluate in ShadowRealms
Should we want to execute code directly in a ShadowRealm, which does not come from another file, we can use the evaluate method on our ShadowRealm, to execute a string of Javascript. This works in much the same way as eval():

```javascript
let myRealm = new ShadowRealm();

myRealm.evaluate(`console.log('hello')`);
```

## ShadowRealm importValue is thennable
Since importValue returns a promise, its value is thennable. That means we can use then() on it, and then do something with the output function that it returns. For example:

```javascript
window.myVariable = 'hello';
let myRealm = new ShadowRealm();

myRealm.importValue('someFile.js', 'createFunction').then((createFunction) => {
    // Do something with createFunction();
})
```

We can also use this methodology to access global variables defined in someFile.js. For example, let's say we changed someFile.js to this:

```javascript
globalThis.name = "fjolt";

export function returnGlobals(property) {
  return globalThis[property];
}
```
Now, in our then function, we could get the value of globalThis.name:

```javascript
window.myVariable = 'hello';
let myRealm = new ShadowRealm();

myRealm.importValue('someFile.js', 'returnGlobals').then((returnGlobals) => {
    // Do something with returnGlobals();
    console.log(returnGlobals("name")); // Returns fjolt
    console.log(window.myVariable); // Returns undefined
})
```

## Conclusion
Today, `iframe`s are the way we usually separate out separate environments on the web. `iframe`s are clunky, and can be quite annoying to work with. ShadowRealms on the other hand, are more efficient, allow us to easily integrate with our existing code base, and integrate well with modern Javascript technologies like [Web Workers](https://fjolt.com/article/javascript-webworkers).

Given their unique value proposition of providing a separated area for code execution, which does not interact at all with the rest of the code base, ShadowRealms will likely become a staple in writing Javascript code. They could become an important way for packages and modules to export their contents without worrying about interference from other parts of the code base. As such, expect to see them popping up in the future.

[Read about the ShadowRealm specification here.](https://github.com/tc39/proposal-shadowrealm/blob/main/explainer.md)