Mutable `shared static` attributes in Javascript

Mon, 11 Mar 2024 00:00:00 +0000

My understanding of the static keyword in JS classes has always been that it determined immutable attributes/methods that are available before not on a class instance but on the class constructor itself. Like with the Object class constrctor: it is possible to instantiate it, but some methods are available on the constructor too, like .fromEntries().

Let’s see a working example:

class MyClass {
 static attr1 = 'attr1'
 attr2 = 'attr2'

 static method1() { return 'method1' }

 method2() { return 'method2' }
}

console.group('Constructor')
console.log(MyClass.attr1)
console.log(MyClass.attr2) // <- doesn't throw, but undefined
console.log(MyClass.method1())
console.log(MyClass.method2()) // <- throws
console.groupEnd()

const instance = new MyClass()
console.group('Instance')
console.log(instance.attr1) // <- doesn't throw, but undefined
console.log(instance.attr2)
console.log(instance.method1()) // <- throws
console.log(instance.method2())
console.groupEnd()

As you can see, static method are available on the constructor only, while “normal” method only on instances. So far so good.

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Mutable shared static attributes in Javascript

Mutable `shared static` attributes in Javascript