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., Boolean, Char, Byte, Short, Int, Long, Float, Double and 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. Any 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 implementation.

If exact single precision operations are important to your application, you can enable strict-floats semantics in Scala.js, with the following sbt setting:

scalaJSLinkerConfig ~= { _.withSemantics(_.withStrictFloats(true)) }

Note that this can have a major impact on performance of your application on JS interpreters that do not support the Math.fround function.

toString of Float, Double and Unit

x.toString() returns slightly different results for floating point numbers and () (Unit).

().toString   // "undefined", instead of "()"
1.0.toString  // "1", instead of "1.0"
1.4f.toString // "1.399999976158142" instead of "1.4"

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, use String.format() or related methods.

Runtime type tests are based on values

Instance tests (and consequently pattern matching) on any of Byte, Short, Int, Float, Double are based on the value and not the type they were created with. The following are examples:

  • 1 matches Byte, Short, Int, Float, Double
  • 128 (> Byte.MaxValue) matches Short, Int, Float, Double
  • 32768 (> Short.MaxValue) matches Int, Float, Double
  • 2147483647 matches Int, Double if strict-floats are enabled (because that number cannot be represented in a strict 32-bit Float), otherwise Int, Float and Double
  • 2147483648 (> Int.MaxValue) matches Float, Double
  • 1.5 matches Float, Double
  • 1.4 matches Double only if strict-floats are enabled, otherwise Float and Double (unlike 1.5, the value 1.4 cannot be represented in a strict 32-bit Float)
  • NaN, Infinity, -Infinity and -0.0 match Float, Double

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. For example, (true: Any).getClass() returns classOf[java.lang.Boolean], not classOf[scala.Boolean].

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. Hence, (5: Any).getClass() will return classOf[java.lang.Byte], while (50000: Any).getClass() will return classOf[java.lang.Integer].

Scala.js 1.x only: Moreover, for () (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 java.lang.Void, as Void is the boxed class corresponding to the void primitive type, which is scala.Unit. 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 Array[scala.runtime.BoxedUnit].

Undefined behaviors

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:

  • NullPointerException
  • ArrayIndexOutOfBoundsException and StringIndexOutOfBoundsException
  • ClassCastException
  • ArrayStoreException
  • StackOverflowError and other VirtualMachineErrors

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.

Scala.js 0.6.x only: In Scala.js 0.6.x, ArithmeticExceptions, such as integer division by 0, are also considered undefined behavior. This is not the case in Scala.js 1.x anymore, where they are reliably thrown.

Some of these, however, can be configured to be compliant with the JVM specification using sbt settings. Currently, only ClassCastExceptions (thrown by invalid asInstanceOf calls) and ArrayIndexOutOfBoundsExceptions (thrown by array indexing) 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 fullOptJS. 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 efficiency.

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 fullOpt stage than in fastOpt.

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 asInstanceOfs:

// Scala.js 1.x
scalaJSLinkerConfig ~= { _.withSemantics(_.withAsInstanceOfs(
  org.scalajs.linker.interface.CheckedBehavior.Compliant)) }

// Scala.js 0.6.x
scalaJSLinkerConfig ~= { _.withSemantics(_.withAsInstanceOfs( }

Note that this will have (potentially major) performance impacts.

JavaScript interoperability

The JavaScript interoperability feature is, in itself, a big semantic difference. However, its details are discussed in a dedicated page.


Java reflection and, a fortiori, Scala reflection, are not supported. There is limited support for java.lang.Class, e.g., obj.getClass.getName will work for any Scala.js object (not for objects that come from JavaScript interop).

Regular expressions

JavaScript regular expressions are slightly different from Java regular expressions. The support for regular expressions in Scala.js is implemented on top of JavaScript regexes.

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 JavaScript does not support weak references, causing all symbols created by Scala.js to remain in memory throughout the lifetime of the application.


The methods Value() and Value(i: Int) on scala.Enumeration use 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:

  1. Calls to either of these two methods of the forms:
    val <ident> = Value
    val <ident> = Value(<num>)
    are statically rewritten to (a slightly more complicated version of):
    val <ident> = Value("<ident>")
    val <ident> = Value(<num>, "<ident>")
    Note that this also includes calls like
    val A, B, C, D = Value
    since they are desugared into separate val definitions.
  2. Calls to either of these two methods which could not be rewritten, or calls to constructors of the protected Val class without an explicit name as parameter, will issue a warning.

Note that the name rewriting honors the nextName iterator. Therefore, the full rewrite is:

val <ident> = Value(
  if (nextName != null && nextName.hasNext)

We believe that this covers most use cases of scala.Enumeration. Please let us know if another (generalized) rewrite would make your life easier.