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> Js barely even supports reflection. This is a huge underrated shortcoming of js if you're doing anything complicated Can you explain what Java offers with re

> Js barely even supports reflection. This is a huge underrated shortcoming of js if you're doing anything complicated Can you explain what Java offers with regard to reflection that JS doesn't? My feeling is that reflection is almost moot in JS since you can inspect/mutate objects at runtime however you like. But maybe I'm missing the point of reflection. There is a Reflect object with a bunch of static methods on it in ES6. It mostly just replicates functonality that already exists in the language, and my suspicion is that it's mainly there so it can be extended at a later date without breaking backwards compatiblity.

spricket 8y ago
Java uses reflection and annotations to support all kinds of language extensions. Js has a lot of nasty issues with the "prototype chain" and annotation support is still experimental. For example, in java, scanning your classes and their annotations to generate code for them at compile time is a standardized feature. Maybe I'm just complaining that JS is too dynamic, but being able to generate code at compile time with reflection and know it will generally work (type checked) is nice.
- kangoo1707 8y ago
  Do you have any concrete example for "For example, in java, scanning your classes and their annotations to generate code for them at compile time is a standardized feature"? Tip: in JS, I don't want OOP class. If you really want class, could you show me the intention behind it?
  - buerkle 8y ago
    http://immutables.github.io/ http://immutables.github.io/ makes great use of this feature in Java
  - gwright 8y ago
    In general this what java annotation processors are all about. As a specific example, Dagger, a compile time, type safe dependency injection framework. https://google.github.io/dagger/ https://google.github.io/dagger/
  - gwright 8y ago
    Retrofit is another example: https://square.github.io/retrofit/ https://square.github.io/retrofit/
  - spricket 8y ago
    Java only works in classes :) . They're actually the compilation unit as well so no way around it. (Files are compiled a .class file at a time from single source files) Adding another example, JPA MetaModel generator that allows you to make strongly typed calls in ORM. Basically, it let's you generate code at compile time that's seamlessly integrated into the existing code. So it's generated code that still gives you IDE autocomplete support etc.
  - stickfigure 8y ago
    Every popular ORM uses reflection to determine how to map fields to columns. For example, what type of date or money object to use. I automagically map some postgres JSONB columns to a parsed object form. Looks just like a regular typed nested object.
    - hajile 8y ago
      That's a solution looking for a problem in JS. You're thinking in types instead of in terms of a dynamic language. You basically HAVE to do this in Java otherwise your program won't work. In JS, you can look everything over at runtime and dispatch from there (made much easier because first-class functions and closures are a thing).
      - stickfigure 8y ago
        you can look everything over at runtime Correct, that's what you have to do in dynamic languages - look at what came in from the database and manually validate+convert it into the format you actually want. Also with pretty much every other form of input (JSON bodies, form posts, queue messages, etc). It's a lot of tedium. When working with node/python/ruby, I often find myself wishing I could just declare a type and be assured that "the system" will get it right instead of me having to code it out myself. You know, like Java. Well-written Java programs are generally more concise and less verbose than well-written JS/Python/Ruby programs. Which is to say, when programmers actually validate input instead of (ahem node) crashing the process and dropping all inflight connections when someone submits a json body missing an expected field. BTW absolutely nothing prevents you from building Java apps by declaring everything as Map<String, Object>. Nobody does that because it's a horrible way to program.
        joshuamorton 8y ago
        > Well-written Java programs are generally more concise and less verbose than well-written JS/Python/Ruby programs. This isn't at all true. I can say this from experience using statically typed Python extensively, in an average code base, 80% of your functions, if not more, are already statically correct in Python, one just need add an annotation. Then you're just as "well written" as the Java. With or without the annotations, the code is still just as "correct". > BTW absolutely nothing prevents you from building Java apps by declaring everything as Map<String, Object>. Nobody does that because it's a horrible way to program Yes, but only because you'd need to explicitly cast things everywhere and write out declarations everywhere. If you could always elide casts and type defs, it becomes a lot less horrible. > Correct, that's what you have to do in dynamic languages - look at what came in from the database and manually validate+convert it into the format you actually want. You still have to do this in static languages. Maybe a library does it for you, and it gives your an error or something you know is a DbRecordFieldStream or whatever, but dynamic languages can do the same thing. Orms validate input in dynamic languages too. Rails and Django can validate request formats. Protobufs work in every language.
orf 8y ago
> Can you explain what Java offers with regard to reflection that JS doesn't? Among hundreds of other things, getting the return type of a method and getting the input parameter names (+ types) in a way that doesn't revolve around literally parsing the functions toString() representation. Oh, and typeof checking that isn't disgustingly broken.
- orangeeater 8y ago
  > Among hundreds of other things, getting the return type of a method and getting the input parameter names (+ types) in a way that doesn't revolve around literally parsing the functions toString() representation. Ah, that's quite cool.
  - orf 8y ago
    You can do almost anything with Java reflection, JS really pales in comparison. Which in some ways is a good thing as you can do a lot more optimizations.
- spricket 8y ago
  Typeof is a good one, but holy crap I had no idea return type and param support was that horrific
- pitaj 8y ago
  > getting the return type of a method and getting the input parameter names (+ types) Type information literally doesn't exist at runtime, though. Isn't it kind of ridiculous to compare languages like that?
  - orf 8y ago
    When you're comparing the relative power of reflection within the language, absolutely not.
    - untog 8y ago
      But what would you use that information for, given that the language has no types? A JS function has no guarantees of what type a function returns anyway. It's like taking issue with the fact that a car doesn't come equipped with skis.
snek 8y ago
Just to be clear, Reflect exists to be direct interface to JS's Module Object Protocol (MOP). Every "primitive" operation you can perform on an object in js, (such as adding a property or getting the value of a property), is exposed via Reflect.
chii 8y ago
There's no easy way to inspect the runtime in js to find all functions or all classes that can be loaded/called. There's no way to hook into the loading of modules in a standard way - you have to know how the modules are compiled and required.
hombre_fatal 8y ago
One example would be a url router for a web server that turns the url variables into the handler's expected argument type and ignores requests that do not. router.get('/users/:id', (id: UUID) => ...) router.get('/products/:id', (id: Int) => ...)
- tracker1 8y ago
  That would be relatively easy to do something very similar with middleware for express/koa router.get('/users/:id'. skipNonUuid(id => ...)) You simply have a function/middleware that does the check.... or `compose(skipNonUuid, dbo.findUserById, ui.formatView('modelName'))` You can do function composition that is nearly as simple and far more flexible in general with JS
est 8y ago
> Can you explain what Java offers with regard to reflection that JS doesn't? One thing comes to mind, how to twinkle stuff inside closures?

spricket 8y ago

Java uses reflection and annotations to support all kinds of language extensions. Js has a lot of nasty issues with the "prototype chain" and annotation support is still experimental. For example, in java, scanning your classes and their annotations to generate code for them at compile time is a standardized feature. Maybe I'm just complaining that JS is too dynamic, but being able to generate code at compile time with reflection and know it will generally work (type checked) is nice.

kangoo1707 8y ago
Do you have any concrete example for "For example, in java, scanning your classes and their annotations to generate code for them at compile time is a standardized feature"? Tip: in JS, I don't want OOP class. If you really want class, could you show me the intention behind it?
- buerkle 8y ago
  http://immutables.github.io/ http://immutables.github.io/ makes great use of this feature in Java
- gwright 8y ago
  In general this what java annotation processors are all about. As a specific example, Dagger, a compile time, type safe dependency injection framework. https://google.github.io/dagger/ https://google.github.io/dagger/
- gwright 8y ago
  Retrofit is another example: https://square.github.io/retrofit/ https://square.github.io/retrofit/
- spricket 8y ago
  Java only works in classes :) . They're actually the compilation unit as well so no way around it. (Files are compiled a .class file at a time from single source files) Adding another example, JPA MetaModel generator that allows you to make strongly typed calls in ORM. Basically, it let's you generate code at compile time that's seamlessly integrated into the existing code. So it's generated code that still gives you IDE autocomplete support etc.
- stickfigure 8y ago
  Every popular ORM uses reflection to determine how to map fields to columns. For example, what type of date or money object to use. I automagically map some postgres JSONB columns to a parsed object form. Looks just like a regular typed nested object.
  - hajile 8y ago
    That's a solution looking for a problem in JS. You're thinking in types instead of in terms of a dynamic language. You basically HAVE to do this in Java otherwise your program won't work. In JS, you can look everything over at runtime and dispatch from there (made much easier because first-class functions and closures are a thing).
    - stickfigure 8y ago
      you can look everything over at runtime Correct, that's what you have to do in dynamic languages - look at what came in from the database and manually validate+convert it into the format you actually want. Also with pretty much every other form of input (JSON bodies, form posts, queue messages, etc). It's a lot of tedium. When working with node/python/ruby, I often find myself wishing I could just declare a type and be assured that "the system" will get it right instead of me having to code it out myself. You know, like Java. Well-written Java programs are generally more concise and less verbose than well-written JS/Python/Ruby programs. Which is to say, when programmers actually validate input instead of (ahem node) crashing the process and dropping all inflight connections when someone submits a json body missing an expected field. BTW absolutely nothing prevents you from building Java apps by declaring everything as Map<String, Object>. Nobody does that because it's a horrible way to program.
      - joshuamorton 8y ago
        > Well-written Java programs are generally more concise and less verbose than well-written JS/Python/Ruby programs. This isn't at all true. I can say this from experience using statically typed Python extensively, in an average code base, 80% of your functions, if not more, are already statically correct in Python, one just need add an annotation. Then you're just as "well written" as the Java. With or without the annotations, the code is still just as "correct". > BTW absolutely nothing prevents you from building Java apps by declaring everything as Map<String, Object>. Nobody does that because it's a horrible way to program Yes, but only because you'd need to explicitly cast things everywhere and write out declarations everywhere. If you could always elide casts and type defs, it becomes a lot less horrible. > Correct, that's what you have to do in dynamic languages - look at what came in from the database and manually validate+convert it into the format you actually want. You still have to do this in static languages. Maybe a library does it for you, and it gives your an error or something you know is a DbRecordFieldStream or whatever, but dynamic languages can do the same thing. Orms validate input in dynamic languages too. Rails and Django can validate request formats. Protobufs work in every language.

orf 8y ago

> Can you explain what Java offers with regard to reflection that JS doesn't? Among hundreds of other things, getting the return type of a method and getting the input parameter names (+ types) in a way that doesn't revolve around literally parsing the functions toString() representation. Oh, and typeof checking that isn't disgustingly broken.

orangeeater 8y ago
> Among hundreds of other things, getting the return type of a method and getting the input parameter names (+ types) in a way that doesn't revolve around literally parsing the functions toString() representation. Ah, that's quite cool.
- orf 8y ago
  You can do almost anything with Java reflection, JS really pales in comparison. Which in some ways is a good thing as you can do a lot more optimizations.
spricket 8y ago
Typeof is a good one, but holy crap I had no idea return type and param support was that horrific
pitaj 8y ago
> getting the return type of a method and getting the input parameter names (+ types) Type information literally doesn't exist at runtime, though. Isn't it kind of ridiculous to compare languages like that?
- orf 8y ago
  When you're comparing the relative power of reflection within the language, absolutely not.
  - untog 8y ago
    But what would you use that information for, given that the language has no types? A JS function has no guarantees of what type a function returns anyway. It's like taking issue with the fact that a car doesn't come equipped with skis.

snek 8y ago

Just to be clear, Reflect exists to be direct interface to JS's Module Object Protocol (MOP). Every "primitive" operation you can perform on an object in js, (such as adding a property or getting the value of a property), is exposed via Reflect.

chii 8y ago

There's no easy way to inspect the runtime in js to find all functions or all classes that can be loaded/called. There's no way to hook into the loading of modules in a standard way - you have to know how the modules are compiled and required.

hombre_fatal 8y ago

One example would be a url router for a web server that turns the url variables into the handler's expected argument type and ignores requests that do not. router.get('/users/:id', (id: UUID) => ...) router.get('/products/:id', (id: Int) => ...)

tracker1 8y ago
That would be relatively easy to do something very similar with middleware for express/koa router.get('/users/:id'. skipNonUuid(id => ...)) You simply have a function/middleware that does the check.... or `compose(skipNonUuid, dbo.findUserById, ui.formatView('modelName'))` You can do function composition that is nearly as simple and far more flexible in general with JS

est 8y ago

> Can you explain what Java offers with regard to reflection that JS doesn't? One thing comes to mind, how to twinkle stuff inside closures?