Semantics of Scala.js
In general, the semantics of the Scala.js language are the same as Scala on the JVM. However, a few differences exist, which we mention here.
Primitive data types
All nine primitive data types of Scala, i.e.,
Unit, work exactly as on the JVM, with the following four exceptions.
Floats can behave as Doubles by default
Scala.js underspecifies the behavior of
Floats by default.
Float value can be stored as a
Double instead, and any operation on
Floats can be computed with double precision.
The choice of whether or not to behave as such, when and where, is left to the
If exact single precision operations are important to your application, you can enable strict-floats semantics in Scala.js, with the following sbt setting:
Note that this can have a major impact on performance of your application on
JS interpreters that do not support
x.toString() returns slightly different results for floating point numbers
In general, a trailing
.0 is omitted.
Floats print in a weird way because they are printed as if they were Doubles,
which means their lack of precision shows up.
To get sensible and portable string representation of floating point numbers,
String.format() or related methods.
Runtime type tests are based on values
Instance tests (and consequently pattern matching) on any of
Double are based on the value and not the
type they were created with. The following are examples:
- 1 matches
- 128 (
> Byte.MaxValue) matches
- 32768 (
> Short.MaxValue) matches
- 2147483647 matches
Doubleif strict-floats are enabled (because that number cannot be represented in a strict 32-bit
- 2147483648 (
> Int.MaxValue) matches
- 1.5 matches
- 1.4 matches
Doubleonly if strict-floats are enabled, otherwise
Double(unlike 1.5, the value 1.4 cannot be represented in a strict 32-bit
As a consequence, the following apparent subtyping relationships hold:
Byte <:< Short <:< Int <:< Double <:< Float <:<
if strict-floats are enabled, or
Byte <:< Short <:< Int <:< Float =:= Double
In Scala/JVM as well as Scala.js, when assigning a primitive value to an
Any (or a generic type), and asking for its
getClass(), Scala returns the boxed class of the value’s type, rather than the primitive value.
(true: Any).getClass() returns
In Scala.js, for numeric types, and for the same reason that instance tests are based on values, the result will be the smallest boxed class that can store the value.
(5: Any).getClass() will return
(50000: Any).getClass() will return
Scala.js 1.x only:
() (unit), the result will be
classOf[java.lang.Void] instead of
classOf[scala.runtime.BoxedUnit] like the JVM.
scala.runtime.BoxedUnit is an implementation detail of Scala on the JVM, which Scala.js does not emulate.
Instead, it uses the more sensible
Void is the boxed class corresponding to the
void primitive type, which is
This means that while
java.lang.Void is not instantiable on the JVM, in Scala.js it has a singleton instance, namely
This also manifests itself in
Array[Unit] which is effectively
Array[java.lang.Void] at run-time, instead of
The JVM is a very well specified environment, which even specifies how some bugs are reported as exceptions. Currently known exhaustive list of exceptions are:
ArithmeticException(such as integer division by 0)
Because Scala.js does not receive VM support to detect such erroneous conditions, checking them is typically too expensive.
Therefore, all of these are considered undefined behavior.
Some of these, however, can be configured to be compliant with the JVM
specification using sbt settings.
ClassCastExceptions (thrown by invalid
are configurable, but the list will probably expand in future versions.
Every configurable undefined behavior has 3 possible modes:
Compliant: behaves as specified on a JVM
Unchecked: completely unchecked and undefined
Fatal: checked, but throws
UndefinedBehaviorErrors instead of the specified exception.
By default, undefined behaviors are in
Fatal mode for
fastOptJS and in
Unchecked mode for
This is so that bugs can be detected more easily during development, with
predictable exceptions and stack traces.
In production code (
fullOptJS), the checks are removed for maximum
UndefinedBehaviorErrors are fatal in the sense that they are not matched by
case NonFatal(e) handlers.
This makes sure that they always crash your program as early as possible, so
that you can detect and fix the bug.
It is never OK to catch an
UndefinedBehaviorError (other than in a testing
framework), since that means your program will behave differently in
stage than in
If you need a particular kind of exception to be thrown in compliance with the
JVM semantics, you can do so with an sbt setting.
For example, this setting enables compliant
Note that this will have (potentially major) performance impacts.
Java reflection and, a fortiori, Scala reflection, are not supported. There is
limited support for
obj.getClass.getName will work
This sometimes has an impact on functions in the Scala library that use regular expressions themselves. A list of known functions that are affected is given here:
StringLike.split(x: Array[Char])(see issue #105)
scala.Symbol is supported, but is a potential source of memory leaks
in applications that make heavy use of symbols. The main reason is that
by Scala.js to remain in memory throughout the lifetime of the application.
Value(i: Int) on
reflection to retrieve a string representation of the member name and
are therefore – in principle – unsupported. However, since
Enumerations are an integral part of the Scala library, Scala.js adds
limited support for these two methods:
- Calls to either of these two methods of the forms:
are statically rewritten to (a slightly more complicated version of):
Note that this also includes calls like
since they are desugared into separate
- Calls to either of these two methods which could not be rewritten,
or calls to constructors of the protected
Valclass without an explicit name as parameter, will issue a warning.
Note that the name rewriting honors the
iterator. Therefore, the full rewrite is:
We believe that this covers most use cases of
scala.Enumeration. Please let us know if another (generalized)
rewrite would make your life easier.