Most users will use DDC via pub. It is supported by pub starting with the Dart 1.24 release.
Soundness and Restrictions
DDC is built upon Dart's new strong mode type system. It only compiles programs that statically type check (i.e., no strong mode errors). It leverages static type checking to generate simpler, readable, and more idiomatic code with fewer runtime checks. In general, DDC is able to provide stronger type guarantees - i.e., soundness - than traditional Dart checked mode with significantly fewer runtime checks.
With strong mode, DDC is stricter than traditional Dart production mode or checked mode. Running existing Dart code on DDC will generally require fixing both static and runtime type errors.
For example, although the following snippet will run in production or checked mode, it will fail to compile with DDC:
var list = ["hello", "world"]; // Inferred as List<String> in strong mode List<int> list2 = list; // Static type error: incompatible types
On the other hand, the following snippet - which tries to mask the type error via casts - will compile with DDC, but fail with a runtime type error.
var list = ["hello", "world"]; List<Object> list2 = list; // Generics are covariant. No runtime check required. List<int> list3 = list2; // Implicit runtime downcast triggers error.
See the strong mode documentation for more details.
During development, a compilation step only needs to be rerun if the Dart files or summaries it relies upon change. For most changes, only a very small part of your code will require recompilation. Morever, modules that are unchanged can be cached in the browser.
Most users invoke DDC indirectly via pub. Pub computes module structure and build steps automatically and invoke DDC accordingly. Pub configures DDC to use AMD modules and uses the standard AMD
require.js loader to bootstrap and load the application.
More advanced users may want to configure or invoke DDC directly. In general, the mapping of Dart files to JS modules is flexible. The key requirement is that module dependences (i.e.,
require in AMD or CommonJS or
import in ES6) must be acyclic. In practice, this means that individual Dart libraries cannot each be mapped to a corresponding JS module (as Dart imports can be and often are cyclic). See the usage document for more details.
There are some import caveats where Dart concepts do not map directly:
- Libraries. Multiple Dart libraries are mapped to a single JS module. Each library appears as a first class object in the generated JS module, with its top-level symbols as members. We currently use a heuristic (based upon file paths) to ensure unique naming of generated library objects.
- Generics. Dart generics are reified, i.e., they are preserved at runtime. Generic classes are mapped to factories that, given one or more type parameters, return an actual ES6 class (e.g.,
HashMap$(core.String, core.int)produces a class that represents a HashMap from strings to ints). Similarly, generic methods are mapped to factories that, given one or more type parameters, return a method.
- Dynamic. DDC supports dynamically typed code (i.e., Dart's
dynamictype), but it will typically generate less readable and less efficient ES6 output as many type checks must be deferred to runtime. All dynamic operations are invoked via runtime helper code.
- Constructors. Dart supports multiple, named and factory constructors for a given class with a different initialization order for fields. Today, these are mapped to instance or static methods on the generated ES6 class.
- Private members. Dart maps private members (e.g., private fields or methods) to ES6 symbols. For example,
a._xmay map to
_xis a symbol only defined in the scope of the generated library.
In general, the current conventions (i.e., the Application Binary Interface or ABI in compiler terminology) should not be considered stable. We reserve the right to change these in the future.
DDC currently supports Chrome stable (though users have had success running on FireFox and Safari). In the near future, we expect to target all common modern browsers that support ES6. ES6 itself is in active development across all modern browsers, but at advanced stages of support: