On Thu, Jan 7, 2016 at 4:55 PM, Alex Harui <aha...@adobe.com> wrote: > How many of you use the "as" keyword as part of a test?
I have a huge code base and I use "as" / "is" everywhere ( https://github.com/WeaveTeam/Weave). My code depends on it behaving the same way it does in ActionScript. IMO, being able to easily check if an object extends or implements a class/interface is a huge advantage AS has over JS. > The reason I'm asking is because the use of "as" as become a negative > factor in several cases: > 1) In JS, it results in a function call > Yes. In ActionScript, using "as" actually sped up the code in certain situations, but in JS it is a performance hit. This is a very common pattern in my code: if (obj is Thing1) (obj as Thing1).foo(a,b); else if (obj is Thing2) (obj as Thing2).bar(x,y); Each pair of is/as results in redundant processing since Language.as() calls Language.is() internally, so I've been changing my code like this to improve performance: var thing1:Thing1 = obj as Thing1; var thing2:Thing2 = obj as Thing2; if (thing1) thing1.foo(a,b); else if (thing2) thing2.bar(x,y); > 2) As Om noted yesterday, it doesn't work for Native JS types > It does work with native types: org.apache.flex.utils.Language.is(document.createElement('button'), HTMLButtonElement) -> true > 3) It causes unnecessary class dependencies which complicates the > goog.requires list > The class dependency is necessary if you need the "as" keyword to behave the same way it does in ActionScript (like I do). > > Currently there is an @flexjsignorecoercion hack you can use to tell > FalconJX to skip code-generation for an "as" operation. However, we are > adding more and more of these, and they are more frequently the cause of > something not working, so I am thinking about flipping the logic and not > generating code for any "as" operation unless you specifically ask for it > via @flexjsgeneratecoercion or something like that. > > Thoughts? > -Alex > > I see "is" and "as" as highly useful features, and I think it would be a mistake to make the code cross-compile into something that behaves differently by default. IMO if cross-compiled code behaves differently than the original then it's being mangled and can't be trusted. A related issue is when setting a typed variable or passing in a parameter to a function, it will do type coercion automatically in AS but that behavior is lost when cross-compiling to JS. For example, I have situations like this where I now have to add manual type casting, and I wish the compiler would do that automatically: var str:String = value_which_may_be_a_number;
