GenerateCode

In this article, we will explore a generic way to create arrays in Scala.js, an important feature especially for developers transitioning from JVM and Scala Native. While the traditional method works seamlessly on the JVM, the same cannot be said for Scala.js due to its different handling of class instances.

Understanding Array Creation in Scala

Creating arrays in Scala typically involves specifying the type and size. The following JVM-compatible code highlights this:

def newArray[A](fqcn: String, size: Int): Array[A] = { val clazz = Class.forName(fqcn) java.lang.reflect.Array.newInstance(clazz, size).asInstanceOf[Array[A]] } 

This code snippet utilizes Java Reflection to create new instances of arrays using their fully qualified class names (FQCN). However, when transitioning to Scala.js, you encounter limitations because java.lang.Class and reflection are not supported.

The Challenge with Scala.js

In Scala.js, you cannot invoke Class.forName as you would in JVM-based Scala due to the lack of runtime reflection capabilities. This raises a fundamental question: How can you achieve a similar outcome?

The answer lies in using Scala.js's own reflective capabilities. Here’s how you can attempt to achieve the same result:

Alternative Array Creation in Scala.js

In Scala.js, you would typically utilize the scala.scalajs.reflect.Reflect object to access or operate on class instances. Here’s a revised version of the original function tailored for Scala.js:

def newArray[A](fqcn: String, size: Int): Array[A] = { val clazz = scala.scalajs.reflect.Reflect.lookupInstantiatableClass(fqcn) .getOrElse(throw new ClassNotFoundException(fqcn)) .runtimeClass java.lang.reflect.Array.newInstance(clazz, size).asInstanceOf[Array[A]] } 

Limitations of the Replacement

Although this code seems promising, it may not work for arbitrary classes unless those classes can be instantiated reflectively. You could face issues if the classes are not visible at runtime due to module encapsulation.

Exploring Generics and Type Safety

When working with generics in Scala.js, consider carefully how type erasure applies. While generics function similarly, the Scala.js compiler may behave differently due to optimizations that avoid certain runtime overheads.

Example Usage of Generic Array Creation

Here’s how you might use the newArray function in practice:

val intArray: Array[Int] = newArray[Int]("java.lang.Integer", 10) val stringArray: Array[String] = newArray[String]("java.lang.String", 5) 

This would create an array of integers with a size of 10 and an array of strings with a size of 5. However, always ensure that the classes you're working with are correctly defined and accessible.

Conclusion

Creating generic arrays in Scala.js can be challenging due to restricted runtime capabilities compared to JVM and Scala Native. Using scala.scalajs.reflect.Reflect you can implement a way to create arrays, although with noted limitations. Keep in mind the possible issues regarding visibility and instantiability of classes in your Scala.js application.

Frequently Asked Questions

How does Scala.js differ from JVM?

Scala.js compiles Scala code to JavaScript, which may limit certain runtime features available in JVM.

Can I use any class in newArray?

Not all classes can be instantiated in Scala.js due to module encapsulation and reflection limitations. Ensure your classes are instantiatable.

What if I face a runtime error related to classes?

Check if the class name is correct and the class is properly instantiated within the scope of your application.