Trapping class definition via Proxy in ES6

Question

Is it possible to trap extends? Or to trap definitions inside a class? Eg:

class B extends A {
    method1( ) { }
    static method2( ) { }
}

Is there any way to trap the events that:

B extended A.
method1( ) was defined on B.prototype
method2( ) was defined on B.

None of the existing mechanisms seem to work. Tried setPrototypeOf and defineProperty traps.

Where do you want to trap these thing? `B` doesn't exist at one moment and is already defined at another. Explaining your case would probably help. — Estus Flask, Jan 15 '17 at 19:57
What problem are you really trying to solve by trapping extend? I'm guessing that there may be other ways to solve it that we could help with. — jfriend00, Jan 15 '17 at 20:29
Use cases are similar to why one would want to trap `defineProperty`: auto-run code when something is defined on an object. Since the `defineProperty` traps weren't triggered, I was checking if `extends` can be trapped. I don't have a strong use case for `extends` trapping otherwise. — Gubbi, Jan 18 '17 at 04:30

score 2 · Accepted Answer · answered Apr 12 '17 at 08:42

2

When class B extends class A, it gets its prototype object. So you can define a Proxy with a trap on get et check if the property being accessed is "prototype".

class A {}

PA = new Proxy(A, {
  get(target, property, receiver) {
    console.log('get', property)
    if (property == 'prototype')
      console.info('extending %o, prototype=%s', target, target.prototype)
    return target[property]
  }
})

class B extends PA {}

answered Apr 12 '17 at 08:42

Supersharp

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@Gubbi note that the prototype could be accessed in other situations so be careful, maybe you'll need to check further depending on your use case. – Supersharp Apr 13 '17 at 15:56

Trapping class definition via Proxy in ES6

1 Answers1