Also, FWIW, since we’re talking about nanoseconds of performance, here, I
think slightly better than:
return this.#random || (this.#random = Math.random());
Is
return (this.#random === undefined) ? this.#random = Math.random() :
this.#random;
This is because in the former, the compiled code has to determine whether
this.#random is coercible into a Boolean. At the very least, it has to do an
"Is this an object and, if so, not undefined ==> true" test. From first
principles, the ternary expression is going to be more efficient. It simply
checks if the value at this.#random is the same as the global undefined value
and, if not, returns that value. No extra tests necessary.
And at least with V8 theory == practice in this case (at least in all my
tests). And yes, we're talking nanosecond here.
While this looks a bit like the null propagation operator would be applicable,
it's really not.
Also FWIW, if you get tired of seeing/typing undefined, we add _ to the global
object which means the above can be written as:
return (this.#random === _) ? this.#random = Math.random() : this.#random;
You might think that this would perform worse than the undefined but, in fact,
for V8 it's identical -- undefined and _ are both just properties of the global
object. I think this reads especially well in function calls where you want an
undefined placeholder
fooey(_, "whatever");
Visually, _ just says undefined to me.
----
Alex Kodat
From: es-discuss [mailto:[email protected]] On Behalf Of Andrea
Giammarchi
Sent: Wednesday, September 13, 2017 2:31 AM
To: Steve Fink <[email protected]>
Cc: [email protected]
Subject: Re: Lazy evaluation
> The properties already existed, so defineProperty shouldn't modify the order
> IIUC
well, nope. the property existed in the prototype, not in the object.
anyway, I guess private properties, that are a possible solution, will be
transpiled through a WeakMap so that most likely anything discussed in here
won't make sense and the future code would look like the following
```js
class A {
#random;
get random() {
return this.#random ||
(this.#random = Math.random());
}
}
// transpiled
var A = function (wm) {
function A() {
wm.set(this, {random: void 0});
}
Object.defineProperties(
A.prototype,
{
random: {
configurable: true,
get: function () {
return wm.get(this).random ||
(wm.get(this).random = Math.random());
}
}
}
);
return A;
}(new WeakMap);
```
_______________________________________________
es-discuss mailing list
[email protected]
https://mail.mozilla.org/listinfo/es-discuss
