On the surface `O instanceof C` boxes `C`, but when using `GetMethod` O turns O into an Object. This means `instanceof` is temporarily turning O into an Object.
For primitives, all the operators in JS have been unable to be altered until this was pointed out. `1 instanceof C` is able to be changed by mutating %NumberPrototype% , `"" instanceof C` by mutating %StringPrototype% , etc. Compared to other operators this is new and a bit surprising that primitive behaviors are mutable. I would expect this behavior by boxing primitives via `Object(primitive) instanceof C` but not for primitives. On Mon, Jun 15, 2015 at 8:43 PM, Allen Wirfs-Brock <[email protected]> wrote: > > On Jun 15, 2015, at 10:06 AM, Bradley Meck wrote: > > Kyle Simpson caused the discovery of this oddity : > > > https://people.mozilla.org/~jorendorff/es6-draft.html#sec-instanceofoperator > calls GetMethod on C without checking if it is an Object, this means `1 > instanceof Foo` could be affected by mutating > `Number.prototype[Symbol.hasInstance]`. > > I am assuming this is unintended? > > > No, it is exactly as intended: > > GetMethod users GetV > http://people.mozilla.org/~jorendorff/es6-draft.html#sec-getv which > handles accessing properties for primitives. For primitive values, it uses > the built-in wrapper prototype to lookup properties, but it does not > actually create a boxed primitive value. > > `this` object boxing is a characteristic of the callee, not the caller. > Non-srict functions box primitive `this values` that are passed to them, > strict mode functions do not. > > This is all the standard method invocation behavior, and the `instanceof` > operator just does a normal method invocation if it finds a > `Symbol.hasInstance` method. Because Function.prototype implements > `Symbol.hasInstacce` all function haves the default legacy behavior when > appearing right-side operand of `instanceof` > > But `instanceof` does not depend upon the existence of a a > `Symbol.hasInstance` method. It falls back to performing the default > (legacy) instanceof check if a callable `hasInstance` property value is > not found. > > But, back to the original question: Exactly what do you expect to be > boxed for `1 instance of Foo`? The `C` that GetMethod is applied to is > `Foo` which would normally be a constructor or some other "type-like" > object implements. It could be a primitive numeric value (eg, `5 > instanceof 4`) which will cause the fallback to be taken using > Number.prototype has a `Symbol.hasInstance` callable property. But, even > if it does why would you expect anything to box? > > Allen > >
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