> On Jul 17, 2015, at 1:14 PM, Travis Leithead <travis.leith...@microsoft.com> > wrote: > > From: Domenic Denicola [mailto:d...@domenic.me] > >>> window.XFoo = document.registerElement(‘x-foo’, XFooStartup); >> >> Why is XFoo different from XFooStartup? If I define a method in XFooStartup, >> does it exist in XFoo? > > This won't work as I described it, given what you've told me, but my > assumption was essentially, that XFooStartup would act as if it didn't really > depend on HTMLElement for construction. So, it's constructor wouldn't be > linked to the actual custom element creation. Therefore XFoo (the > platform-provided constructor function is the thing that is actually used to > trigger construction, which would then result in the XFooStartup constructor > running. Basically, like this (reverting to non-class syntax): > > function XFooStartup(val1, val2) { > this.prop = val1; > this.prop2 = val2; > } > window.XFoo = document.registerElement(‘x-foo’, XFooStartup); > > all I was trying to express different from the current design is: > 1) replacing the createdCallback with the function constructor (and passing > the new element instance as 'this' when calling it) > 2) passing through params from the XFoo platform-native constructor to the > XFooStartup function > 3) calling XFooStartup synchronously
We can do this without wrapping author supplied constructor. In ES6/ES2015 classes, `this` variable is in the temporary dead zone (TDZ) until `super()` which allocates `this` and any attempt to access it will throw `ReferenceError`. In other words, XFooStartup has no way of accessing the newly constructed object until `super()` has returned. This in turns allows browser engines to create a native (C++) backing store for the HTML element inside HTMLElement’s constructor (or an equivalent code that runs as a part of call to `super()` from the direct subclass of HTMLElement) since the newly constructed (this) element is never accessed until the highest superclass' `super()` (which is HTMLElement in this case) had been called. - R. Niwa
