In general, the semantics of the Scala.js language are the same as Scala onthe JVM.However, a few differences exist, which we mention here.

Primitive data types

All primitive data types work exactly as on the JVM, with the following threeexceptions.

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 onFloats can be computed with double precision.The choice of whether or not to behave as such, when and where, is left to theimplementation.

If exact single precision operations are important to your application, you canenable 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 onJS interpreters that do not supportthe Math.fround function.

toString of Float, Double and Unit

x.toString() returns slightly different results for floating point numbersand () (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 thetype 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

otherwise.

Undefined behaviors

The JVM is a very well specified environment, which even specifies how somebugs are reported as exceptions.Currently known exhaustive list of exceptions are:

• NullPointerException
• ArrayIndexOutOfBoundsException and StringIndexOutOfBoundsException
• ClassCastException
• ArithmeticException (such as integer division by 0)
• StackOverflowError and other VirtualMachineErrors

Because Scala.js does not receive VM support to detect such erroneousconditions, checking them is typically too expensive.

Therefore, all of these are consideredundefined behavior.

Some of these, however, can be configured to be compliant with the JVMspecification using sbt settings.Currently, only ClassCastExceptions (thrown by invalid asInstanceOf calls)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 throwsUndefinedBehaviorErrorsinstead of the specified exception.

By default, undefined behaviors are in Fatal mode for fastOptJS and inUnchecked mode for fullOptJS.This is so that bugs can be detected more easily during development, withpredictable exceptions and stack traces.In production code (fullOptJS), the checks are removed for maximumefficiency.

UndefinedBehaviorErrors are fatal in the sense that they are not matched bycase NonFatal(e) handlers.This makes sure that they always crash your program as early as possible, sothat you can detect and fix the bug.It is never OK to catch an UndefinedBehaviorError (other than in a testingframework), since that means your program will behave differently in fullOptstage than in fastOpt.

If you need a particular kind of exception to be thrown in compliance with theJVM semantics, you can do so with an sbt setting.For example, this setting enables compliant asInstanceOfs:

scalaJSLinkerConfig ~= { _.withSemantics(_.withAsInstanceOfs(
  org.scalajs.core.tools.sem.CheckedBehavior.Compliant)) }

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

JavaScript interoperability

The JavaScript interoperability feature is, in itself, a big semanticdifference. However, its details are discussed in adedicated page.

Reflection

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

Regular expressions

JavaScript regular expressionsare slightly different fromJava regular expressions.The support for regular expressions in Scala.js is implemented on top ofJavaScript regexes.

This sometimes has an impact on functions in the Scala library thatuse regular expressions themselves. A list of known functions that areaffected is given here:

• StringLike.split(x: Array[Char]) (see issue #105)

Symbols

scala.Symbol is supported, but is a potential source of memory leaksin applications that make heavy use of symbols. The main reason is thatJavaScript does not support weak references, causing all symbols createdby Scala.js to remain in memory throughout the lifetime of the application.

Enumerations

The methods Value() and Value(i: Int) on scala.Enumeration usereflection to retrieve a string representation of the member name andare therefore – in principle – unsupported. However, sinceEnumerations are an integral part of the Scala library, Scala.js addslimited support for these two methods:

• 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.

• Calls to either of these two methods which could not be rewritten,or calls to constructors of the protected Val class without anexplicit name as parameter, will issue a warning.Note that the name rewriting honors the nextNameiterator. Therefore, the full rewrite is:

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

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