Glimmer.js Progress Report

– By Tom Dale

At EmberConf in March of this year, we announced Glimmer.js, a library for building modern UI components optimized for the mobile web. I wanted to give an update on what we've been working on since then.

There were two primary motivations for releasing Glimmer.js as a standalone project:

  1. We wanted people who aren't "all in" on Ember to have a way to incrementally adopt part of the framework.
  2. We wanted a laboratory where we could freely run experiments on what a next-generation component library might look like, without creating churn for Ember users.

Because Ember's rendering layer is built on top of the shared Glimmer VM, successful experiments have a clear path to make their way upstream to Ember users. And, once stabilized, we'd like Glimmer.js to be the default component API for new Ember apps—but it's still too premature to set any timelines for that today.

Unlocking Experimentation

While Ember is known for incorporating the best ideas from across the JavaScript ecosystem, it's important that we contribute back our own innovations, too. Refining new ideas takes time and, often, a few false starts. How do we reconcile the need to experiment with Ember's vaunted stability guarantees?

One of the themes we discussed in our keynote was a new focus on unlocking experimentation, that is, allowing people to easily try out and share unproven ideas on top of the stable Ember core.

Unlocking experimentation doesn't just allow for more ideas; it also leads to better ideas, because you can try things without worrying about breaking changes. With Glimmer.js, we wanted to eat our own experimental dogfood.

We've wanted to overhaul the component API in Ember for some time now. But because components play such a central role, we knew that we had to have a tangible implementation for people to play with before we could credibly ask them to comment on an RFC. And we knew that having an implementation would almost certainly shake out problems with the design that we would need to address.

Glimmer.js is our way to iterate on a new component API until we have something we feel confident submitting to the Ember RFC process.

We've already received incredibly useful feedback from early adopters. In addition, fellow core team member Chad Hietala and I have been on a team at LinkedIn using Glimmer.js to build a production application.

To quote DHH:

The best frameworks are in my opinion extracted, not envisioned. And the best way to extract is first to actually do.

If you notice that a lot of the items I describe below are performance-related, that is at least partly due to our product's all-consuming focus on mobile load times. We are extremely excited about some of the recent breakthroughs we've made and have enjoyed proving out some of our more esoteric ideas in a real app.

What's New in Glimmer

We've been really, really busy and I've got a ton of updates to share with you. This blog post got pretty long, so here's the short version:

  1. We're adopting <Capital />-style component syntax.
  2. We're adding support for customizing a component's DOM attributes with ...attributes.
  3. You'll be able to teleport your component's elements anywhere in the DOM with the built-in {{in-element}} helper.
  4. We've now got what we think might just be the fastest component library in the running, with:
  5. Compiled binary bytecode
  6. Incremental rendering
  7. SSR with incremental rehydration

Adopting <Capital /> Components

One of the most eagerly-awaited features of Glimmer.js is "angle bracket components," or components that you invoke <like-this /> instead of {{like-this}}. I personally really like this syntax because it visually disambiguates components from the dynamic data that flows through them. It also unifies the attribute syntax between HTML and components:

{{! Ember }}
{{my-button title=title label=(t "Do Something")}}

{{! HTML }}
<button title={{title}} label={{t "Do Something"}}></button>

{{! Glimmer.js (today) }}
<my-button @title={{title}} @label={{t "Do Something"}} />

This syntax is how Glimmer.js works today. However, the more we discussed the design and started using it in real projects, the more we believed that this exact API was flawed and needed to be rethought.

First, a short history lesson. When we introduced components in Ember (back before Ember 1.0!), we required them to include a dash (-) in their name. This rule came from the then-new Custom Elements spec, a key part of the Web Components API.

Web Components have the problem of needing to disambiguate between built-in elements and custom elements. What happens if I make a component called <vr></vr> and later the browser adds a built-in Virtual Reality element with the same name?

The compromise was that custom elements must have a dash, keeping single-word elements reserved for future versions of the HTML standard.

Of course, Ember components don't have the same problem, because they use {{ and }} as delimeters instead of < and >. Nonetheless, we preemptively adopted this constraint because we assumed Web Components were going to take the world by storm and at some point we would need to migrate Ember components to Web Components.

As time has passed, though, it's become increasingly clear that the use cases served by Web Components, wonderful as they are, do not have the full set of functionality to replace everything that an Ember component (or React component, etc.) needs to do.

Meanwhile, the more components I write, the more grating the naming restriction feels. I hate the cognitive overhead of having to invent silly names like {{x-button}} when a single word would be much more descriptive.

Unfortunately, this puts us in a bit of a pickle:

  1. We want to drop the annoying dasherized-component requirement.
  2. We want to adopt <angle> brackets syntax for components, but now we have the same naming collision hazard as Web Components.
  3. People want to use Web Components in their apps, so how do we know if <my-button> means "create a custom element" or "create a Glimmer component"?

The core team has circled around different designs for months, and this topic has dominated both our weekly calls and our in-person meetings.

At the most recent face-to-face meeting in Austin, we finally reached consensus on a proposal that I'm really excited about.

How do you disambiguate between Glimmer components and HTML elements? Our proposal is to borrow the same rule that React uses: components always start with a capital letter.

Our above example turns into this:

{{! new Glimmer.js }}
<Button @title={{title}} @label={{t "Do Something"}} />

I love this for a few different reasons.

First, to me, the capital letter makes components really stand out in the template, and we can improve syntax highlighting in editor plugins to make it stand out even more. It also makes it clear when you're invoking a Web Component or not, whereas the original Glimmer.js syntax was ambiguous.

Second, I hate the friction of having to invent a two-word name. The app I work on has at least three different conventions people use to prefix or suffix components. It feels bad and makes templates look more noisy than they should. When we switched from dasherized to capitals, it felt viscerally like an improvement.

Third, for better or worse, many people consider React to be an "industry standard" and aligning component naming makes Glimmer templates feel that much more familiar. (Although do note that we are just adopting the naming convention, not JSX itself!)

This change also helps us solve the problem of "fragment" or "tagless" components, i.e., templates that don't have a single root element.

While we've supported this in Ember for a long time, we were nervous about the potential for confusion if you typed something like <my-button /> in a template, which looks like a Web Component, and it didn't always correlate to a single element in the DOM.

Today in Glimmer.js, it is a compile-time error if your component template doesn't have a single root element. With <Capital> components, we will remove this restriction and allow you to have whatever you want in your template.

One side-effect of this is that we will replace this.element (which today is a reference to the component's root element) with this.bounds.firstNode and this.bounds.lastNode, allowing you to traverse the range of DOM nodes belonging to your component.

There are still some open questions about what, if any, sugar to provide for the single-element case.

For example, we could make this.element available in just those cases, although we're concerned that someone coming along and adding an extra element to a template could subtly break any code relying on this.element. Another proposal was to set this.element to the element with ...attributes on it (see below). We're looking forward to more design and discussion about how to make this ergonomic without being error-prone.

Component Attributes

Without getting into a full explanation of the difference between properties and attributes in the DOM, suffice it to say that most web developers have a muddy mental model at best. (And rightfully so—it took me forever to understand the difference.)

While you can get pretty far pretending properties and attributes are interchangeable, eventually you are going to run into cases where you really have to set a property or you really have to set an attribute.

Server-side rendering (SSR) complicates the issue because, of course, HTML can only serialize attributes, not properties.

One drawback of both Ember and React's components is that they don't do a great job of making it easy for consumers of components to set attributes.

Let's take React as an example (although, again, it applies just as much to Ember). I want to write a reusable HiResImage component that anyone can install from npm and use in their apps. It wraps an <img> element and renders a low-resolution image by default, swapping in a high-resolution image when clicked.

// HiResImage.jsx
import { Component } from 'react';

export default class HiResImage extends Component {
  render(props, state) {
    let showHiRes = () => { this.setState({ hiRes: true }); }
    let { src, hiResSrc } = props;
    let { hiRes } = state;

    return <img src={hiRes ? hiResSrc : src} onClick={showHiRes}>;

Now we can use the component like this:

<HiResImage src="corgi.jpg" hiResSrc="corgi@2x.jpg" />

But what happens if I want to set the width of the underlying img element via its width attribute?

<HiResImage width="100%" src="corgi.jpg" hiResSrc="corgi@2x.jpg" />

This won't work because the only attributes or properties that get set are the ones we've manually listed in our render() method! The width attribute here will just get ignored.

We have a few options, but none of them are that great.

  • We could enumerate all of the possible valid img attributes that might get passed in, but that is error-prone (new attributes get added all the time) and takes a lot of code.
  • We could use the spread operator (...props), but that will set everything passed in as an attribute, even "known" props that aren't attributes, like hiResSrc.
  • If we're using Babel, we can use rest syntax in object destructuring to separate "known" and "unknown" props: let { src, hiResSrc, ...attrs } = props. But this requires non-trivial runtime work, and means any props passed in by accident will now be treated as an attribute.
  • In Preact, the problem is even trickier because it will always set the width property no matter what, never the attribute. And setting the width property to "100%" results in an image zero pixels wide.

With Glimmer.js, you explicitly disambiguate between properties and attributes via the presence of the @ sigil. In symmetry with HTML, attributes do not have @, while component arguments (props in React parlance) do:

<HiResImage @src="corgi.jpg" @hiResSrc="corgi@2x.jpg" width="100%" />

In this example, src and hiResSrc are JavaScript values passed as arguments to the component object, and width is serialized to a string and set as an attribute.

"But wait," you ask, "if components aren't required to have a single root element anymore, where do attributes go?"

With recent changes in Glimmer VM, we can now support an ...attributes syntax that we've colloquially started calling "splattributes" (because they "splat" attributes from the outside onto an element).

In our case, the Glimmer.js version of the HiResImage component might look like this:

// component.js
import Component, { tracked } from '@glimmer/component';

export default class HiResImage extends Component {
  @tracked hiRes = false;

  showHiRes() {
    this.hiRes = true;
{{!-- template.hbs --}}
{{#if hiRes}}
  <img src={{@hiResSrc}} ...attributes>
  <img src={{@src}} {{on click=(action showHiRes)}} ...attributes>

Here, any attributes passed in on the invoking side will be "splatted" onto the appropriate element.

So what happens if you try to pass attributes to a component that doesn't have ...attributes? At runtime, you'll get a hard error telling you that the component should add ...attributes to one or more elements. We can probably produce compile-time errors in the majority of less dynamic cases, too.


Typically, the component hierarchy maps directly to the DOM hierarchy, meaning all of a component's elements are rendered inside a DOM element that belongs to the parent component.

Occasionally, though, it can be helpful to break out of the current DOM tree and render a component's content somewhere else. While there are many different use cases, the most common one I've seen is for rendering modal dialogs.

This is easy to do now with the built-in {{in-element}} helper. This helper will render the block you pass to it inside a foreign element. (In React-land, this functionality is usually referred to as a portal, and as of React 16 is included by default in react-dom.)

For example, if I had a Modal component and I wanted to always render its content into a specially-styled element at the root of the body (with position: fixed, say), I might write it like this:

// component.js
import Component from '@glimmer/component';

export default class Modal extends Component {
  modalElement = document.getElemenyById('modal');
{{!-- template.hbs --}}
{{#in-element modalElement}}

Now in my app, I can invoke my Modal component as deep into the hierarchy as I want, and the content will be rendered into the root modal element:

{{!-- template.hbs --}}
{{#if hasErrors}}
    <p>You dun goofed:</p>
      {{#each errors as |error|}}

Binary Templates

It's crucial that web apps render instantly, or else users go elsewhere. When it comes to improving web performance, one of the most frequent recommendations you'll hear is to minimize the amount of total JavaScript in your app.

There are two reasons for this: not only does more JavaScript take longer to download, just parsing the JavaScript can become a noticeable bottleneck on underpowered devices.

Complicating the advice to "use less JavaScript" is the fact that most modern JavaScript libraries, including Angular, React, Vue and Svelte, compile component templates (or JSX) to JavaScript, which gets embedded in application code. Without aggressive hand optimization, more templates means more JavaScript.

Ember used to do the same thing, compiling Handlebars templates into JavaScript code that would first create and then update a component's DOM tree.

With Glimmer, however, we took a different approach. Instead of generating JavaScript, today we compile templates into a JSON data structure of "opcodes," or rendering instructions. A small runtime evaluates these opcodes, translating them into DOM creation, DOM updates, invocation of component hooks, etc.

Not only is a JSON parser much faster than a full-blown JavaScript parser, aggressively sharing code in the Glimmer VM generates less on-device memory pressure and allows JavaScript engines like V8 to more quickly generate JIT-optimized code.

Best of all, our compact JSON format is significantly smaller than the equivalent compiled JavaScript. We received many reports of apps dropping 30-50% in total (post-gzip!) size after upgrading to Ember 2.10, the first version to use this JSON-based approach.

As exciting as this was, we knew that JSON was not the final word in compactly and efficiently representing compiled templates.

At runtime, the Glimmer VM today gathers the JSON for each template and compiles them into a final, internal representation that is just a large array of 32-bit integers. After looking at traces of real-world Glimmer.js apps, we knew we could improve boot times by precomputing this final compilation step at build time.

Helpfully, browsers have become increasingly fluent at dealing with binary data, largely driven by demanding multimedia use cases like audio, video, and 3D graphics. And while JSON is fast to parse, as the old saw goes, no parse is faster than no parse. What if we could serialize compiled templates into a binary format that the VM could start executing without a parse step?

I'm no M. Night Shyamalan, so you've probably already guessed the ending here: that's exactly what we've done. Recent versions of Glimmer VM include the @glimmer/bundle-compiler package, our name for the compiler that produces a binary "bundle" of all of your compiled templates.

We are planning to land support for binary templates as an opt-in in Glimmer.js soon. (The feature is already landed in the low-level Glimmer VM but is not yet exposed in a convenient way.)

One thing to note about the bundle compiler is that it requires knowing your entire program statically at build time. The browser tends to be a pretty dynamic environment, however, so Glimmer VM still supports "lazy compilation" (i.e. compiling to JSON) as a first-class mode.

In the Ember ecosystem, apps and addons do very dynamic things (like register components at runtime) which are incompatible with the bundle compiler. We want to enable binary templates in Ember, but this is farther out because we will need to figure out exactly what the constraints are and provide guidance for app and addon authors.

In exchange for the (admittedly pretty incredible) performance benefits, binary templates also introduce extra complexity.

Binary templates can't be inlined in HTML or JavaScript, so they must be fetched as early as possible in the page lifecycle. No browser I tested yet supports <link rel="preload" as="fetch">, which would allow a streaming HTML parser to detect and fetch binary data very early in the page load. No tools or CDNs know what the heck a .gbx file is (the file extension of compiled binaries), and require manual configuration. You probably want H2 Push for this, but that's its own can of worms.

Getting these optimally deployed will probably be painful for a while, but I have faith that the Ember community will do what it does best and rally around a set of shared, high quality solutions for dealing with this.

If you're curious about the details of how binary templates work, don't miss Chad's recent post about the optimizing compiler.

Server-Side Rendering

Server-side rendering, or SSR, is a technique for rendering your components on the server. It allows you to send meaningful HTML to a user's browser so that they see something other than an empty white rectangle before your JavaScript finishes loading.

Ember supports SSR through FastBoot, and Glimmer VM is Ember's rendering engine, so you can probably guess that Glimmer already has support for SSR, too.

That's true, but there are two shortcomings we want to address:

  1. Running Glimmer.js apps in SSR mode is not as easy as it should be.
  2. Today we take a performance hit because of how we serialize to HTML.

To address the first problem, we are going to make SSR a first-class API and document exactly how you go from writing a Glimmer.js app to connecting it to a Node HTTP server.

Second, after looking at profiles of our app running in SSR mode, we noticed that there is some low-hanging fruit to pick in how we generate HTML.

Glimmer is able to run in Node because, internally, we use an abstraction for building and modifying the DOM. Instead of calling document.createElement(tagName) directly, for example, an opcode might instead call this.dom.createElement(tagName), going through a DOM construction helper.

In the browser, this just proxies to document.createElement, but in Node, we instead use simple-dom, a lightweight implementation of a small subset of the DOM API—a "virtual DOM," if you will. Once rendering is complete, we use simple-dom's built-in serializer to convert the DOM tree to HTML and send it over the wire.

This approach has the huge advantage of keeping the DOM mutable, just like in the browser. Particularly with FastBoot, where we wanted existing Ember apps to be able to adopt SSR, this was an important compatibility feature.

The downside to preserving mutability is that it introduces a performance double-whammy.

HTML is immutable, in the sense that once I write <div>, there's nothing I can write later in the file to go back and add an attribute to that element. If the DOM is mutable, then we have to wait for the entire document to settle before we can serialize to HTML and start writing to the HTTP socket.

This delays the time to first byte (TTFB) by at least the time it takes to render the entire page, even though many of the components may have finished rendering hundreds of milliseconds previously.

The other performance hit is that we now have two (often large) trees to traverse: first we walk the tree of components in the initial render, and then we have to walk DOM tree during HTML serialization. This is predominantly a CPU cost, but allocating all of these temporary "virtual DOM" nodes doesn't help memory costs, either.

Chad has been doing some experiments with implementing a version of our DOM abstraction that writes HTML directly to a stream. This solves both of the above-mentioned performance pitfalls nicely.

By writing directly to the stream, the host Node environment can start flushing bytes to the browser immediately in a background thread. And by writing strings directly to the stream buffer, we can avoid allocating intermediate data structures entirely.

For this to work, we have to introduce two requirements:

  1. Components can only render once on the server, so any data fetching needs to happen before the initial render starts.
  2. Internally, we need to order opcodes to align with HTML; that is, we need to ensure that we always create an element and set its attributes before attempting to append any child nodes. (Luckily for us, it happened to work this way already.)

We are hoping to test this approach in a real app in the next few weeks and report back on the results. We are tentatively optimistic that this will result in significant performance improvements, and suspect it may outperform even the best SSR implementations of Virtual DOM-based libraries like React and Preact because it requires fewer allocations.


Rehydration is the ability of a client-side JavaScript app to "reconnect" components to the DOM generated by server-side rendered HTML. In many ways, a robust rehydration implementation is the holy grail that reunites the progressive enhancement and "all JavaScript all the time" camps.

The server can serve up meaningful HTML, viewable even when JavaScript isn't available, and that HTML gets "progressively enhanced" with interactivity once the JavaScript loads and components rehydrate. But because components can also still produce their own DOM, you retain all of the benefits of client-side JavaScript, including the ability to work offline.

Glimmer supports rehydration natively via its ElementBuilder abstraction. In SSR mode, you can enable the SerializeBuilder, which includes additional comment annotations for where dynamic sections start and end.

For example, given this template:

<span class="user__name">{{}}</span>

The serialized output would include comments indicating the dynamic portion:

<span class="user__name"><!--%+block:0%-->Chad Hietala<!--%-block:0%--></span>

In the browser, Glimmer is configured to use the RehydrateBuilder. The RehydrateBuilder treats the existing DOM as a stack, and as the VM requests new elements get created, it compares the top of the "DOM stack" to the requested element. If it matches, the element is reused and the DOM is not mutated at all. In the case of a mismatch, the current block is cleared and the appropriate element is created and put in its place. The comment annotations are also stripped during this process, so you see a pristine DOM in your developer tools.

We have rehydration working in our app, and we consider it to be a first-class part of the Glimmer SSR story.

Incremental Rendering

The thing about low-end mobile devices is that no matter how much you optimize, performing the initial render of most modern web applications is going to be slow. While it's important to chase the fastest raw performance possible (and we certainly have with Glimmer!), on some devices it's just unavoidable to have initial renders that take 500ms to over a second.

This produces a terrible user experience because it blocks the main thread. If the user happens to be scrolling, their scroll suddenly starts to stutter. Videos and animated GIFs freeze, the browser stops updating the layout, and everything generally feels "janky."

The problem is that, by dominating the main thread for so long, we're not being good citizens of the web. The CPU is a shared resource, and we're hogging it all for ourselves for a huge chunk of time.

But what if we had a way to render a few components at a time, periodically giving control back to the browser so it could handle scrolling, painting, etc.?

Glimmer's architecture is actually perfectly suited for this, because at the end of the day it's just executing a linear sequence of operations. We can execute each opcode, one by one, and pause execution if it starts taking too long.

Best of all, because normal rendering and rehydration go through nearly identical code paths, this technique applies whether you're creating fresh DOM or just rehydrating DOM created from server-rendered HTML.

Incremental rehydration feels amazing, even on low-end devices on slow networks. The server sends complete HTML, which the browser can incrementally parse and render, even before CSS and JavaScript have started loading. Once the JavaScript does finish loading, it can rehydrate arbitrarily complex DOM while maintaining 60fps scrolling, never taking more than ~16ms before returning execution to the browser.

We're using requestIdleCallback in our app to drive rehydration, which provides a "deadline" describing how much time we have to do work before causing user-noticeable jank. We execute opcodes until we hit the deadline, then schedule the VM to resume in another idle callback if there are additional operations to execute. For browsers without requestIdleCallback, we can fall back to setTimeout and approximate deadlines.

Wrapping Up

Except where noted, everything I've described above is available in raw, low-level form in Glimmer VM today. We will be releasing a new version of Glimmer.js imminently that includes the updated VM, including the change to <Capital /> components, fragment templates, etc.

Once that's done, our next task will be to make enabling binary templates, SSR, rehydration, etc. as easy as possible.

We are also actively working on making Glimmer components available in Ember apps via an addon, as well as updating Ember to use the latest version of Glimmer VM. Once that lands, it should unlock the ability to use rehydration in FastBoot.

I hope this overview has got you as excited as I am. Glimmer's VM architecture has been the gift that keeps on giving, and I've been surprised by how relatively easy implementing the above features has been on top of the core architecture.

Best of all, I don't think we've yet hit the end of the possibilities that Glimmer unlocks. In this post, I've focused on features that are either done or close to being done, and haven't yet mentioned some of the ideas we're excited to try, like running Glimmer in a Web Worker, porting the core VM to WebAssembly, and more. We've also been working closely with our browser implementer friends to see what lessons can be applied to the web platform so everyone benefits from our experimentation.

Lastly, I'd like to thank LinkedIn and Tilde for funding a great deal of the implementation work. Not only is all of this work released under the open source MIT license, we do all of our work out in the open on GitHub. I invite you to follow along on the Glimmer VM and Glimmer.js repositories.

Thank you so much for reading this far, and I can't wait to get all of this cool stuff into your hands. We are looking forward to seeing what the community can build with these powerful primitives. We'll post again once we've released the next version of Glimmer.js with these features integrated, so stay tuned to the blog. And if you've got any questions or want to help out, leave a comment below or come see us on GitHub!