Is there a Class equivalent to prototype attributes?

Question

If I have the following code:

function Thing() {}
Thing.prototype.cache = {};

var a = new Thing();
var b = new Thing();

a.cache === b.cache // true

How might I re-write this using proper Classes?

I know about public class fields, but those are defined on the instance rather than on the class prototype.

// For Example:
class Thing {
  cache = {}
}

var a = new Thing();
var b = new Thing();

a.cache === b.cache // false

Answer 1

There's no class syntax to put non-methods on the prototype, you'll have to slap them on afterwards, like this:

class Thing {}
Thing.prototype.cache = {}

// Example usage //

const thing1 = new Thing()
const thing2 = new Thing()
thing1.cache.x = 2
console.log(thing2.cache.x)

However, putting mutable data on the prototype is seen by many as magical and difficult to follow. Here's another way to achieve a very similar effect.

// This can alternativly be a static, private variable on the class.
const thingCache = {}

class Thing {
  cache = thingCache
}

// Example usage //

const thing1 = new Thing()
const thing2 = new Thing()

thing1.cache.x = 2
console.log(thing2.cache.x)

But, what I would really recommend is to just make this property a static property on the class - that's what static properties are for, sharing a single piece of data between many instances.

class Thing {
  static cache = {}
}

// Example usage //

Thing.cache.x = 2
console.log(Thing.cache.x)

// In some rare scenarios where you only have the instance,
// and not the original class, you can still retrieve the original class

const thing1 = new Thing()
const thing2 = new Thing()

thing1.constructor.cache.x = 2
console.log(thing2.constructor.cache.x)