DDevelopment & Design

Not so easy functional programming in JavaScript

Introduction

JavaScript allows for operating on arrays in a functional way, e.g. using filter or map functions. As an argument for these functions we can pass lambda expression or function reference. Is there a difference between them? The answer is yes.

What’s the problem?

In our project we are building a mapping using String.fromCharCode function. To simplify the usage of this function looked similar to:

[66, 67, 68].map(v => String.fromCharCode(v))

When we run this code with node we received [ 'B', 'C', 'D' ], but when we decided to refactor it to use function reference the result was different:

> [66, 67, 68].map(String.fromCharCode)
[ 'B\u0000\u0000', 'C\u0001\u0000', 'D\u0002\u0000' ]

What happened?

To find the reason for this behavior, let’s first play with function String.fromCharCode alone:

> String.fromCharCode(66)
'B'
> String.fromCharCode([66, 67, 68])
'\u0000'
> String.fromCharCode(66, 67, 68)
'BCD'

String.fromCharCode deals with various types and numbers of arguments.

Now, let’s examine the function map:

> [66, 67, 68].map(v => v)
[ 66, 67, 68 ]
> [66, 67, 68].map((v, u) => [v, u])
[ [ 66, 0 ], [ 67, 1 ], [ 68, 2 ] ]
> [66, 67, 68].map((v, u, w) => [v, u, w])
[ [ 66, 0, [ 66, 67, 68 ] ],
  [ 67, 1, [ 66, 67, 68 ] ],
  [ 68, 2, [ 66, 67, 68 ] ] ]

map, like many other array functions, passes always three arguments to function. First is the current value, the second is the index of the current value and third is the whole array.

It means that passing String.fromCharCode to map function under the hood looks like this:

> [66, 67, 68].map((v, u, w) => String.fromCharCode(v, u, w))
[ 'B\u0000\u0000', 'C\u0001\u0000', 'D\u0002\u0000' ]

and it is equal to the initial example.

Conclusion

We must be careful when we want to use a function that can take more than one argument, but we want to pass only the value. We have to pass the function as a lambda expression:

> [66, 67, 68].map(v => String.fromCharCode(v))
[ 'B', 'C', 'D' ]

or create another function which ensures that only the first argument will be passed to desired function:

> const useOnlyValue = f => v => f(v);
undefined
> [66, 67, 68].map(useOnlyValue(String.fromCharCode))
[ 'B', 'C', 'D' ]

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DDevelopment & Design

Grails render as JSON catch

One of a reasons your controller doesn't render a proper response in JSON format might be wrong package name that you use. It is easy to overlook. Import are on top of a file, you look at your code and everything seems to be fine. Except response is still not in JSON format.

Consider this simple controller:

class RestJsonCatchController {
def grailsJson() {
render([first: 'foo', second: 5] as grails.converters.JSON)
    }

def netSfJson() {
render([first: 'foo', second: 5] as net.sf.json.JSON)
    }
}

And now, with finger crossed... We have a winner!

$ curl localhost:8080/example/restJsonCatch/grailsJson
{"first":"foo","second":5}
$ curl localhost:8080/example/restJsonCatch/netSfJson
{first=foo, second=5}

As you can see only grails.converters.JSON converts your response to JSON format. There is no such converter for net.sf.json.JSON, so Grails has no converter to apply and it renders Map normally.

Conclusion: always carefully look at your imports if you're working with JSON in Grails!

Edit: Burt suggested that this is a bug. I've submitted JIRA issue here: GRAILS-9622 render as class that is not a codec should throw exception

DDevelopment & Design

Inconsistent Dependency Injection to domains with Grails

I've encountered strange behavior with a domain class in my project: services that should be injected were null. I've became suspicious as why is that? Services are injected properly in other domain classes so why this one is different?

Constructors experiment

I've created an experiment. I've created empty LibraryService that should be injected and Book domain class like this:

class Book {
    def libraryService

    String author
    String title
    int pageCount

    Book() {
        println("Finished constructor Book()")
    }

    Book(String author) {
        this()
        this.@author = author
        println("Finished constructor Book(String author)")
    }

    Book(String author, String title) {
        super()
        this.@author = author
        this.@title = title
        println("Finished constructor Book(String author, String title)")
    }

    Book(String author, String title, int pageCount) {
        this.@author = author
        this.@title = title
        this.@pageCount = pageCount
        println("Finished constructor Book(String author, String title, int pageCount)")
    }

    void logInjectedService() {
        println("    Service libraryService is injected? -> $libraryService")
    }
}
class LibraryService {
    def serviceMethod() {
    }
}

Book has 4 explicit constructors. I want to check which constructor is injecting dependecies. This is my method that constructs Book objects and I called it in controller: 

class BookController {
    def index() {
        constructAndExamineBooks()
    }

    static constructAndExamineBooks() {
        println("Started constructAndExamineBooks")
        Book book1 = new Book().logInjectedService()
        Book book2 = new Book("foo").logInjectedService()
        Book book3 = new Book("foo", 'bar').logInjectedService()
        Book book4 = new Book("foo", 'bar', 100).logInjectedService()
        Book book5 = new Book(author: "foo", title: 'bar')
        println("Finished constructor Book(Map params)")
        book5.logInjectedService()
    }
}

Analysis

Output looks like this:

Started constructAndExamineBooks
Finished constructor Book()
    Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2
Finished constructor Book()
Finished constructor Book(String author)
    Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2
Finished constructor Book(String author, String title)
    Service libraryService is injected? -> null
Finished constructor Book(String author, String title, int pageCount)
    Service libraryService is injected? -> null
Finished constructor Book()
Finished constructor Book(Map params)
    Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2

What do we see?

  1. ✔

    Empty constructor injects dependencies.

  2. ✔

    Constructor that invokes empty constructor explicitly injects dependencies.

  3. ✘

    Constructor that invokes parent's constructor explicitly does not inject dependencies.

  4. ✘

    Constructor without any explicit call declared does not call empty constructor thus it does not inject dependencies.

  5. ✔

    Constructor provied by Grails with a map as a parameter invokes empty constructor and injects dependencies.

Conclusion

Always explicitily invoke empty constructor in your Grail domain classes to ensure Dependency Injection! I didn't know until today either!