[BUGFIX] Use WeakMap to store tracked property values. #18091
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pzuraq
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Jun 13, 2019
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Using a `Symbol` causes issues on IE11 when using the core-js polyfill.
Specifically, the polyfill adds every symbol to `Object.prototype` and
therefore our `if (symbol in this) {` check was completely borked.
Migrating to a `WeakMap` exposed another issue: we were accidentally
relying on the fact that the symbols would be inherited through the
prototype chain. This meant that setting a tracked property on a
prototype would actually affect the value seen by every instance of that
prototype.
For example:
```js
class Thing {
@Tracked baz;
}
Thing.prototype.baz = 'foo';
let thing = new Thing();
console.log(thing.baz)
```
Before the changes in this commit, the above console.log would emit
`foo` and after these changes it would print `undefined`.
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Using a
Symbolcauses issues on IE11 when using the core-js polyfill. Specifically, the polyfill adds every symbol toObject.prototypeand therefore ourif (symbol in this) {check was completely borked.Migrating to a
WeakMapexposed another issue: we were accidentally relying on the fact that the symbols would be inherited through the prototype chain (introduced by #18074). This meant that setting a tracked property on a prototype would actually affect the value seen by every instance of that prototype.For example:
Before the changes in this commit, the above
console.logwould emitfooand after these changes it would printundefined.Fixes #18075