Glimmer.js: What’s the Deal with TypeScript?

Two weeks ago at EmberConf, we announced Glimmer.js, a component-based library for writing superfast web applications.

In the demo video, we use TypeScript to write our Glimmer components. Some people have been asking, what’s the deal? Have we turned our backs on JavaScript and embraced our new TypeScript overlords?


I’m not usually a fan of “alternative” JavaScript. All the way back in 2013, for example, I was arguing against adopting CoffeeScript. It was easy to see that there was important new functionality coming in ES6 and beyond, and it wasn’t clear how CoffeeScript users would be able to take advantage of it.

For example, CoffeeScript adopted for..of loops to iterate over the properties of an object. Meanwhile, ES6 introduced its own for..of loops as a way to loop through iterable objects. If CoffeeScript wants to support new functionality with similar syntax like this, it has two choices: break existing CoffeeScript code, or diverge from JavaScript more and more over time.

Because JavaScript is the lingua franca of the web, and the web is everywhere, it is a programming language subject to enormous compatibility constraints. The smallest language change can potentially render millions of sites inoperable.

I have witnessed the intense amount of thought and care TC39 members devote to figuring out how to introduce modern language features in a way that is completely backwards-compatible. It involves dumping out all of the existing features and syntax on a table, and painstakingly moving them around until a compatible path can be traced through.

Unfortunately, this heroic effort only helps JavaScript, not “alt-JavaScript.” At the end of the day, transpiled languages suffer from one of two problems:

  1. “JavaScript-alikes” like CoffeeScript will have to break their own backwards compatibility, or diverge from JavaScript (and thus become less JS-alike over time).
  2. Languages with totally different syntax and semantics, like ClojureScript, are difficult to debug, even when source maps are working perfectly.

But not TypeScript. TypeScript is different.


TypeScript is different because of how radically constrained it is.

Most transpiled languages exist because they want to correct some perceived deficiencies in JavaScript. It’s hard to quantify, but you can feel the respect that the TypeScript team has for JavaScript. They’re not trying to rescue an inherently flawed language; instead, they’re trying to help a language they love reach new heights.

I love TypeScript because the delta between it and JavaScript is so small. In fact, you can take any JavaScript file, change the extension to .ts, and boom, you’ve got a valid TypeScript file.

Because TypeScript is a superset of JavaScript, you’ve actually been writing TypeScript this whole time.

From there, you can incrementally add type checking only where you think it’s necessary.

It’s easy for people to have a visceral negative reaction to TypeScript. Let’s be frank: a lot of TypeScript examples are nearly indistinguishable from Java.

But two points:

  1. All of that extra type syntax is optional; only use it if it’s bringing you benefits.
  2. Don’t get fooled into thinking that TypeScript is as awkward and occasionally frustrating to use as Java. Behind that Java-like syntax is a language that is every bit as flexible and dynamic as JavaScript because, well, it is JavaScript.

JavaScript + Types = TypeScript

For people who haven’t used TypeScript, it’s easy to imagine that it might be a bunch of weird, complicated new stuff on top of JavaScript, in addition to types. In reality, TypeScript is just JavaScript plus the smallest possible set of syntax additions required to let you incrementally typecheck your code.

What’s incredible about the TypeScript compiler is that it doesn’t transpile code so much as just strip out type annotations. Debugging is straightforward because each line of TypeScript corresponds to the same line of JavaScript, just without the types.

Here’s an example TypeScript file, and the resulting compiled JavaScript (targeting ES2017):

As you can see, we’re using cutting-edge ES2017 features like async functions, and the syntax is exactly the same across both. The only difference in the TypeScript version is that we declare the fields on the Person class, as well as the type of the arguments to the constructor; these simply disappear in the JavaScript output.

And even these simple annotations quickly begin to pay dividends. For example, using a TypeScript-enabled editor like VS Code gives us detailed information about class properties, just by hovering our mouse over them:

“But I Still Don’t Want to Use TypeScript!”

And that’s totally cool! Glimmer is a library, first and foremost, for writing JavaScript apps. If you don’t want to use TypeScript, you should have the freedom not to.

“That’s what Angular said and look how that turned out.”

OK, I guess this is maybe the elephant in the room. A lot of people’s first exposure to TypeScript was Angular 2. Looking at the Angular website, it’s easy to get the sense that using TypeScript means JavaScript becomes a de facto second-class citizen.

For example, here’s an Angular component in TypeScript:

Here’s the same thing in ES6 JavaScript:

As you can see, the JavaScript version is quite a bit longer than the TypeScript version. So why wouldn’t Glimmer suffer the same fate?

The primary reason is that Angular relies on an experimental TypeScript feature that emits type information in the compiled JavaScript (i.e., emitDecoratorMetadata). Dependency injection is front and center in Angular, so they (very rationally) decided to use the types you’d write anyway to power the DI system.

I actually think this is a cool example of improving developer ergonomics in Angular by using existing type information, but it does have the unfortunate effect of requiring different, often awkward APIs for people who want to use JavaScript.

Glimmer goes in the other direction. We like that TypeScript is just JavaScript, so the APIs you use in both languages are exactly the same. Because Glimmer is written in TypeScript, you get great autocompletion and type checking out of the box, but there’s no requirement to use TypeScript, and there’s no alternate set of APIs for JavaScript.


After having used TypeScript for nearly a year, I have to confess: I never want to start a new project without it again.

Refactors that used to take weeks take days, sometimes less. And because refactoring is so much easier, cleanup that would never have happened becomes… almost painless.

In literally every case, converting a project from JavaScript to TypeScript has identified at least one bug. Because I’m now aware of the kinds of bugs TypeScript catches automatically, writing code without it feels like walking a tight rope without a net.

Most of all, I continue to be impressed by the professionalism of the TypeScript team. In an ecosystem that can feel built on a house of unmaintained cards, TypeScript’s drumbeat of constant, iterative improvement is refreshing.

TypeScript is exploding in popularity and I think it will continue to only get more popular. As I finish writing this post, Felix Rieseberg’s post on using TypeScript at Slack is on the front page of Hacker News. I suspect that most people who try TypeScript will end up liking it.

At the end of the day, though, JavaScript is the language of the web. It’s our job to give you a library that feels great whatever you choose.

That’s why we design our APIs for JavaScript first, and thanks to TypeScript’s love for JavaScript, it couldn’t be easier.

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