Scala.js

Correctness

Performance

Interoperability

Excellent editor support

With Scala.js, typos and type-errors are immediately caught and shown to you in your editor, without even needing to compile your code. Refactor any field or method with ease, with the confidence that if you mess it up the editor will tell you immediately. Stop flipping back and forth between your editor and MDN, because your editor will display what methods are available, what arguments they take, what they return, and even their documentation, right in-line with your code!

class Person {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  fullName() {
    return `${this.firstName} ${this.lastName}`;
  }
}

class Person(val firstName: String, val lastName: String) {
  def fullName(): String =
    s"$firstName $lastName"
}

const names = persons.map(p => p.firstName);

val names = persons.map(p => p.firstName)
// or an even shorter version
val names = persons.map(_.firstName)

const personMap = new Map([
  [10, new Person("Roger", "Moore")],
  [20, new Person("James", "Bond")]
]);
const names = [];
for (const [key, person] of personMap) {
  if (key > 15) {
    names.push(`${key} = ${person.firstName}`);
  }
}

val personMap = Map(
  10 -> new Person("Roger", "Moore"),
  20 -> new Person("James", "Bond")
)
val names = for {
  (key, person) <- personMap
  if key > 15
} yield s"$key = ${person.firstName}"