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This FAQ applies to web programming. For more general Dart questions, see the FAQ, or the Tools FAQ, both on dartlang.


Q. What browsers do you support as JavaScript compilation targets?

We support the following browsers:

  • Internet Explorer, versions 10 and 11
    • Dart v1.5 was the last release to support Internet Explorer 9.
  • Firefox, latest version
  • Chrome, latest version
  • Safari for desktop, latest version
  • Safari for mobile, latest version

Q. Is Dart supported by my browser?

Although no production browsers can execute Dart code directly, all modern browsers can execute Dart code that’s been compiled to JavaScript. For convenience while you’re developing Dart code, you can use a version of Chromium (nicknamed Dartium) has the Dart VM integrated into it.

Q. How do I debug an app once it has been compiled to JavaScript?

Dart web apps are easiest to debug with an IDE, like WebStorm, and Dartium. However, the debugging section of the dart2js documentation has some tips for specific browsers.

Q. Can I use Angular with Dart?

Yes! AngularDart is a port of Angular to Dart.

Q. Can I use web components with Dart?

Yes! Polymer Dart is a port of polymer to Dart. Polymer is a set of polyfills and usability enhancements for web components.

Q. Should I use Angular or Polymer?

Both. Or either. It’s really up to you.

Q. Can I build a Chrome App with Dart?

You can use the chrome.dart libraries, but you still need to compile to JavaScript. But otherwise, yes, you can build a Chrome App with Dart.

JavaScript and other web technologies

Does Google want to replace JavaScript with Dart?

We believe developers should have a choice when they build for the web. Adding a new option, such as Dart, does not imply replacing an existing option.

Q. Isn’t Dart a lot like JavaScript?

Yes and no. The Dart project thinks that JavaScript can use some changes for more productive software engineering, smarter editors and development environments, and web apps that are as beautiful and pleasing as the best client apps can be. On the other hand, we don’t think everything needs to change, and why change what isn’t broken?

Dart, like JavaScript, is a dynamically typed language. It adds optional type annotations to help you catch errors earlier. It takes out a few features of JavaScript, such as prototypes and the global object: this streamlines the VM, enables faster execution, and makes it easier to do code completion and refactoring. And Dart adds some goodies. To name a few:

  • User-defined operator methods. We like the lightweight, readable code these give for our DOM interface.

  • Lightweight syntax for anonymous functions. You use them a lot in web programming; now they look great. And they come with correct binding of this and full block-level lexical scoping, no gotchas.

Dart is more than a new syntax, it’s a full language with its own semantics. Dart differs from JavaScript in many ways, including:

  • Only true is true.
  • No undefined, only null.
  • No automatic type coercion with ==, +, and other operators.

When compared to JavaScript, Dart aims to be faster, more regular, and more scalable to large programs.

Q. How does Dart code interoperate with JavaScript libraries?

Although Dart and JavaScript are completely separate languages with separate VMs, they can interoperate. For more information, see package:js and the chartjs example.

Q. I have a large JavaScript codebase. How can I migrate it to Dart?

Try migrating one major feature at a time, and use the JavaScript interoperability library only when necessary.

Q. How does Dart compare with using the Closure compiler on JavaScript?

The idea of optional type annotations is similar. Dart’s are nicer syntactically.

Compare the following Closure compiler code:

// Closure compiler code

 * @param {String} name
 * @return {String}
makeGreeting = function(name) {
  /** @type {String} */
  var greeting = 'hello ' + name;
  return greeting;

With the following Dart code:

// Dart code

String makeGreeting(String name) {
  String greeting = 'hello $name';
  return greeting;

Q. How does Dart compare with CoffeeScript?

Both Dart and CoffeeScript are inspired by JavaScript, and both can be translated back to it. They make different choices, particularly in the flavor of their syntax. As a language we think it’s fair to say that Dart differs semantically from JavaScript more than CoffeeScript does; that may result in a less line-for-line translation, but we believe Dart-generated JavaScript can have excellent size and speed.

Dart introduces new semantics, while CoffeeScript retains the semantics of JavaScript.

If you like CoffeeScript for its more structured feel than raw JavaScript, you may like Dart’s optional static type annotations.

Q. What does Google think of TypeScript?

TypeScript and Dart have similar goals; they make building large-scale web applications easier. However, their approaches are fairly different. TypeScript maintains backwards compatability with JavaScript, whereas Dart purposely made a break from certain parts of JavaScript’s syntax and semantics in order to eradicate large classes of bugs and to improve performance. The web has suffered from too little choice for too long, and we think that both Dart and TypeScript are pointing to a brighter future for web developers. You can read a more complete response on our blog.

Q. I have a large application written in GWT. How do I port it to Dart?

Java and Dart are syntactically similar, so this might be easier than you think. You can rely on the Dart analyzer to flag any syntax problems. Alternatively, you may consider porting one feature at a time to Dart and using the JavaScript interoperability library as the common middle ground. Be sure to watch our Google I/O 2012 talk Migrating Code from GWT to Dart, but keep in mind that it predates our JavaScript interoperability library.

JavaScript compilation

Q. Will any valid Dart code compile to JavaScript, or are there limitations?

We intend for any valid Dart code to compile to JavaScript. Of course, there are some libraries that will only run on the server because they don’t make sense in a browser context. For example, the dart:io library provides access to operating system files and directories with APIs not available to the browser.

Q. How can dart2js produce JavaScript that runs faster than handwritten JavaScript?

Think of dart2js as a real compiler, which can analyze your entire program and make optimizations that you probably can’t or won’t do. Just like gcc can output efficient code by moving code around, dart2js can take advantage of Dart’s structured nature to implement global optimizations.

We don’t claim that all Dart code will run faster than handwritten JavaScript, when compiled to JavaScript, but we’re working to make the common cases fast.

Q. How can I write Dart code that compiles to performant JavaScript?

See Helping dart2js generate better code. Just be aware that this information might change as the implementation of dart2js changes.

Q. Why is the code for “Hello, World” so big, compared to the original Dart code after compilation to JavaScript?

We believe that it’s important to create small and efficient JavaScript from Dart, but most developers don’t write “Hello, World” apps. It’s all relative, and with tree shaking (dead code elimination), minification, and compression, Dart apps can be compiled to JavaScript fairly efficiently.

Kevin Moore saw improvements in the size of the generated JavaScript from his real-world HTML5 game.

The dart2js team strives to generate smaller output, but is more focused on real-world apps instead of trivial examples.

Q. How are floating point numbers handled when compiled to JavaScript?

JavaScript has only one number representation: an IEEE-754 double-precision floating-point number. This means that any number—integer or floating point—is represented as a double. JavaScript has typed data arrays, and the mapping from native Dart typed lists to JavaScript typed arrays is trivial.

Q. How are integers handled when compiled to JavaScript?

Because all numbers are stored as doubles, integers are restricted to a 53-bit precision. Integer values in the range of -253 to 253 can be stored without loss of accuracy. Because JavaScript VMs play tricks with the internal representation of numbers (similar to those described above), staying within smi range is still good practice.

Q. How are typed lists handled when compiled to JavaScript?

JavaScript offers typed arrays that are compatible with Dart’s typed lists. The mapping is trivial—for example, Float32List becomes a Float32Array. The one exception today is that dart2js does not support 64-bit integers and thus does not support Int64List or Uint64List. Dart code compiled via dart2js results in a runtime exception if either of those lists is used.


Q. Why Dart?

At Google we’ve written our share of web apps, and we’ve tried in many ways to make improvements to that development process, short of introducing a new language. Now we think it’s time to take that leap. We designed Dart to be easy to write development tools for, well-suited to modern app development, and capable of high-performance implementations.

Q. Is the language what really needs to be fixed in web development?

We want to fix ALL the things. There’s “Dart” the language, and then there’s “Dart” the overall project. The Dart project is betting that the language needs some changes, but we also want to improve the DOM and other libraries, and to improve the tools we use.

At the same time, Google is also placing bets that JavaScript can be evolved as needed, and contributing to that work. Google wants web development to be great, and if that happens with JavaScript, we’re happy.

Q. Is Dart going to divert community effort from JavaScript-based web development?

If people like Dart and use it, then to a certain extent, yes, but isn’t this true of any improvement to existing web development? Nothing is zero-effort to learn or 100% back-compatible to legacy browsers, so people use both new and old. You might look at it this way: Google is putting significant effort behind both Dart and JavaScript, choosing to develop Dart while at the same time using JavaScript extensively, and working on JavaScript tools, implementation, and language spec. We’re doing both because we think Dart is worth it.

Server-side web programming finds room for many languages: does Python divert effort from Perl, and does Java undercut C++? Again, to a certain extent, yes they do, but people generally consider that a healthy situation, better than if we all used a single programming language. Multiple languages have allowed for faster change than any single language has achieved through a standards process. Furthermore, languages coexist in different niches: does Groovy really compete directly with C++? People face different engineering tradeoffs and choose different languages to meet them. Ultimately, we think client-side developers should have this kind of flexibility.

Q. Will the Dart VM get into Chrome?

No. Dart is designed to compile to JavaScript to run across the modern web, and the dart2js compiler is a top priority for the team.

Q. Why doesn’t Dart support IE9 or earlier?

Supporting legacy browsers takes a lot of engineering resources and testing infrastructure. Dart is a bet for the future, and the project can’t push forward if it needs to spend valuable resources on supporting browsers that are dying or dead. Also, dart2js can emit efficient code if it assumes a modern browser with ECMAScript5 or greater features.

Q. Why not compile Dart to asm.js instead of building a specialized VM?

Dart could have used asm.js in two ways; compiling Dart applications to asm.js, or compile the Dart VM to asm.js.

However, after careful consideration it becomes clear that both ways incur non- acceptable overhead which nullifies some of Dart’s value proposition: its fast start-up and better performance.

Compilation of a Dart application to asm.js

Asm.js is a very restricted subset of JavaScript best suited as a compilation target for C compilers. It does not include JavaScript objects, or direct access to the DOM. Essentially, it only allows arithmetic operations and manipulations on typed arrays.

While it is possible to implement the dynamic features that are required by Dart, they would incur a large overhead in both speed and size, compared to relying on the already existing features provided by the underlying JS engine. For example, any JS machine comes with a garbage collector (henceforth GC), and implementing another one in asm.js would increase the output size, and be noticeably slower than the well-tuned GCs of modern JS VMs.

Similarly, JS VMs have spent significant effort in making dynamic dispatch efficient, using a combination of dynamic code generation and self-modifying code.

Compilation of the Dart VM to asm.js (for example via emscripten)

Arguments in the preceding section also apply here. A Dart VM in asm.js would need to reimplement, on top of asm.js, many facilities that are already provided by the JS VMs. Furthermore, asm.js doesn’t allow direct access to all machine capabilities, like threading and specialized instruction sets.

Shipping the Dart VM (compiled to asm.js) with every program would also add significant download size to every Dart program. Even cached, it would still take a long time to compile the Dart VM (as asm.js) program on the client, yielding significant start-up times.

Furthermore you would have to rewrite the Dart VM backend to generate asm.js code, as the Dart VM relies on dynamic code generation to achieve peak performance. (In an additional step, the JS VM would then need to compile that code into assembly, adding to the latency.)

The generated code would be restricted to the instruction set that is provided by asm.js, whereas a native VM can emit specialized instructions for the platform.

That said, it would be amazing to see the Dart VM compiled to asm.js. This experiment would have little practical value, but it would be a nice achievement.