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

Private fields and methods are not private in groovy

I used to code in Java before I met groovy. Like most of you, groovy attracted me with many enhancements. This was to my surprise to discover that method visibility in groovy is handled different than Java!

Consider this example:

class Person {
private String name
public  String surname

private Person() {}

private String signature() { "${name?.substring(0, 1)}. $surname" }

public String toString() { "I am $name $surname" }
}

How is this class interpreted with Java?

  1. Person has private constructor that cannot be accessed
  2. Field "name" is private and cannot be accessed
  3. Method signature() is private and cannot be accessed

Let's see how groovy interpretes Person:

public static void main(String[] args) {
def person = new Person()   // constructor is private - compilation error in Java
println(person.toString())

    person.@name = 'Mike'       // access name field directly - compilation error in Java
println(person.toString())

    person.name = 'John'        // there is a setter generated by groovy
println(person.toString())

    person.@surname = 'Foo'     // access surname field directly
println(person.toString())

    person.surname = 'Bar'      // access auto-generated setter
println(person.toString())

println(person.signature()) // call private method - compilation error in Java
}

I was really astonished by its output:

I am null null
I am Mike null
I am John null
I am John Foo
I am John Bar
J. Bar

As you can see, groovy does not follow visibility directives at all! It treats them as non-existing. Code compiles and executes fine. It's contrary to Java. In Java this code has several errors, pointed out in comments.

I've searched a bit on this topic and it seems that this behaviour is known since version 1.1 and there is a bug report on that: http://jira.codehaus.org/browse/GROOVY-1875. It is not resolved even with groovy 2 release. As Tim Yates mentioned in this Stackoverflow question: "It's not clear if it is a bug or by design". Groovy treats visibility keywords as a hint for a programmer.

I need to keep that lesson in mind next time I want to make some field or method private!

DDevelopment & Design

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Nasi koledzy z Fluidcircle opublikowali właśnie studium przypadku opisujące doświadczenia TouK z wykorzystania metodyki Scrum. Wszystko w ramach programu “Scrum…