Documented breaking change for subclassing built-ins. #100
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mhegazy
approved these changes
Dec 6, 2016
| you may find that: | ||
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| * methods may be `undefined` on objects returned by constructing these subclasses, so calling `sayHello` will result in ane error. | ||
| * `instanceof` will be broken between instances of the subclass and their instances, so `(new FooError()) instanceof FooError` may return `false`. |
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| As part of substituting the value of `this` with the value returned by a `super(...)` call, subclassing `Error`, `Array`, and others may no longer work as expected. | ||
| This is due to the fact that constructor functions for `Error`, `Array`, and the like use ES6's `new.target` to adjust the prototype chain; | ||
| however, there is no way to emulate the semantics of `new.target` in ECMAScript 5. |
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it might be worth noting that it was not working correctelly in TS 2.0 either for the same reason.
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I disagree with the statement that "there is no way to emulate the semantics of new.target", we just haven't implemented it yet.
Example:
class C {
constructor() {
return Object.setPrototypeOf({}, new.target.prototype);
}
}
//
var C = (function () {
function C() {
var _newTarget = this && this instanceof C ? this.constructor : void 0;
return Object.setPrototypeOf({}, _newTarget.prototype);
}
return C;
})();There was a problem hiding this comment.
I'll amend this so that we state that there is no way to ensure a value of new.target when invoking a constructor.
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@mhegazy @rbuckton