You are correct, it is not the primitive being boxed, but the primitive being passed into @@hasInstance.
Still consider it very strange that I can change if `1 instanceof Number` but if that was intentional /shrug. Typeof I guess will be the safe way to detect things. On Tue, Jun 16, 2015 at 12:53 AM, Allen Wirfs-Brock <[email protected]> wrote: > > On Jun 15, 2015, at 9:21 PM, Bradley Meck wrote: > > 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. > > > Read the spec > http://people.mozilla.org/~jorendorff/es6-draft.html#sec-relational-operators-runtime-semantics-evaluation > > > `o instanceof C` performs InstanceOfOperator(O,C) which performs > GetMethod(C, @@hasInstance) > > so GetMethod doesn't even see the value of `o`. > > > 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. > > > In ES6 certain operations (instanceof and the internal ToPrimitive which > is used by several operator) were made extensible because Host and built-in > objects have always been able to extend their semantics. For example, > WebIDL has it's own unique definition of instanceof that it uses for DOM > objects. Extensibility of these operations via JS code is necessary in > order to enable such libraries to be directly self-hosted using JS code. > > > 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. > > > O is the argument to hasInstance methods. JS has never auto-boxed > primitive argument values. > > Allen > > >
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