Commits on Apr 25, 2021

1. lib: add support for JSTransferable as a mixin …

   Adds a new `makeTransferable()` utility that can construct a
   `JSTransferable` object that does not directly extend the
   `JSTransferable` JavaScript class.
   
   Because JavaScript does not support multiple inheritance, it is
   not possible (without help) to implement a class that extends
   both `JSTransferable` and, for instance, `EventTarget` without
   incurring a significant additional complexity and performance
   cost by making all `EventTarget` instances extend `JSTransferable`...
   
   That is, we *don't* want:
   
   ```js
   class EventTarget extends JSTransferable { ... }
   ```
   
   The `makeTransferable()` allows us to create objects that are
   backed internally by `JSTransferable` without having to actually
   extend it by leveraging the magic of `Reflect.construct()`.
   
   ```js
   const {
     JSTransferable,
     kClone,
     kDeserialize,
     kConstructor,
     makeTransferable,
   } = require('internal/worker/js_transferable');
   
   class E {
     constructor(b) {
       this.b = b;
     }
   }
   
   class F extends E {
     [kClone]() { /** ... **/ }
     [kDeserialize]() { /** ... **/ }
   
     static [kConstructor]() { return makeTransferable(F); }
   }
   
   const f = makeTransferable(F, 1);
   
   f instanceof F;  // true
   f instanceof E;  // true
   f instanceof JSTransferable;  // false
   
   const mc = new MessageChannel();
   mc.port1.onmessage = ({ data }) => {
     data instanceof F;  // true
     data instanceof E;  // true
     data instanceof JSTransferable;  // false
   };
   mc.port2.postMessage(f);  // works!
   ```
   
   The additional `internal/test/transfer.js` file is required for the
   test because successfully deserializing transferable classes requires
   that they be located in `lib/internal` for now.
   
   Signed-off-by: James M Snell <jasnell@gmail.com>

   

   jasnell committed Apr 25, 2021
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