Is Javascript a Functional Programming Language?

Just because functions are first class objects, there are closures, and higher order functions, does Javascript deserve to be called a Functional Programming language? The main thing I think it lacks is Pure Functions, and it doesn't 'feel' like other functional languages, like lisp (although thats not really a good reason for it not to be a functional langauge...)

Answers:

Answer

Repeating my own answer to a similar question,

There's no accepted definition of functional programming language.

If you define functional language as the language that supports first class functions and lambdas, then yes, JavaScript *is* a functional language.

If you also consider the factors like support for immutability, algebraic data types, pattern matching, partial application etc then no, JavaScript *is not* a functional language.


I'd encourage you to read the following related blog posts (and also the comments below them):

Answer

I would say that it is a multi-paradigm language.

EDIT: It's multi-paradigm and includes functional constructs.

Answer

if you stretch and twist the term "functional programming" to the point of philosophical discussions, this question may be open again. However, then you end up on the level of useful questions like "Is C++ really a programming language"?

The answer to your question on more daily level is "no".

Functional programming means that the program is conceptualized as a evaluation of a function, rather than a control flow. The code is a description of functions, and has no inherent concept of a control flow.

JavaScript has got a control flow and is conceptualized as a imperative language. From its design objective, it is clearly not a functional language.

Answer

The term "functional programming" language is so overloaded these days it's almost useless. There are two dominant meanings:

  1. Has first-class functions
    • Javascript is this!
  2. Is based on functions as used in the lambda calculus, with an emphasis on avoiding persistent mutable state (often replacing it with parameters passed to functions)
    • As commonly written, Javascript is not remotely this!

Pick your meaning and then the question is answerable.

Answer

I don't think there a concrete definition of functional programming , however many of things people consider "functional programming" can be done with javascript. Here is a good brief example in this article.

Answer

To me, Javascript is both an imperative language and a functional language, and you can choose to use it either way, and even (egad) both ways. Or you can choose to use one paradigm and never touch the other. It's up to you. I, like you, don't think Javascript should be called a Functional Language, because it allows you to wander in and out of the functional programming paradigm. Perhaps if it had a pragma of some kind, to limit you using only functional programming paradigms, then that would be useful, I think. But, in summary, I say it's more of a imperative/procedural language with some functional programming features tossed in.

Answer

I tend not to think of programming languages as having one particular paradigm, but that they lend themselves to certain paradigms. However just because they lend themselves to a particular paradigm doesn't mean you have to use that paradigm. It's quite possible to write object oriented programs in C and write imperative programs in ML. Not using a certain paradigm to solve a problem because the language isn't designed for it is just artificially limiting yourself (of course you should still take into account the limitations of a language when deciding if a particular solution will be a good solution).

Answer

Well, I wouldn't say it's functional programming, but then I would say it's object oriented and just today a friend said he wouldn't put it on that shelf either.

So, while I wouldn't say it is, I guess there's room for opinion. It does have classical features of functional programming, it doesn't have others.

Answer

Javascript is to a point. It truly depends on how you go about programming it. If I code in an OO manner, would it not be OO? So if you just code things in a 'functional' manner it would be functional. I guess it is multi-paradigm language so to call it just one thing isn't entirely accurate.

Answer

@petraszd I rewrite your code a little to obtain a "new" for operator:

   
   function ffor(a, b, f){
     function it(i){
       if(i > b)return
       f(i)
       it(i+1)
     }
     it(a)
   }

   print("----" + new Date()+"----")

   var funcs = []
   ffor(0, 9, function(i){
     funcs.push(function(){return i})
   })

   ffor(0, 9, function(i){
     print(funcs[i]())
   })

But I know that this way has disadvantages for big loops...

Related question about tail recurtion optimization in JS

P.S. Posted here cuz have problem with code formatting while posting as comment

Answer

In Javascript, you can do something like this!!

// Data
var fruits = [
    { name: 'apple',  price: 5 }, 
    { name: 'orange', price: 10 }, 
    { name: 'lemon',  price: 15 }
]

// Request Data from magicURL
request('magicURL')
    .then(selectKeyOf('price'))
    .then(priceMethod('sum'))
    .then((result)=>{
        console.log(result) // 30
    })

I have made a github page in order to demo this concept and you can clone/view my implementation

Answer

As we know the functional programming language doesn't allow to change or mutate the elements(state)of functions but in javascript it is allowed in that sense it is not a functional programming language, although it does treat function as first class citizens.

Answer

What I really hate in javascript (if You try to look at it as FP language) is this:

function getTenFunctionsBad() {
  var result = [];
  for (var i = 0; i < 10; ++i) {
    result.push(function () {
      return i;
    });
  }
  return result;
}

function getTenFunctions() {
  var result = [];
  for (var i = 0; i < 10; ++i) {
    result.push((function (i) {
      return function () {
        return i;
      }
    })(i));
  }
  return result;
}

var functionsBad = getTenFunctionsBad();
var functions = getTenFunctions()
for (var i = 0; i < 10; ++i) {
  // using rhino print
  print(functionsBad[i]() + ', ' + functions[i]());
}

// Output:
//   10, 0
//   10, 1
//   10, 2
//   10, 3
//   10, 4
//   10, 5
//   10, 6
//   10, 7
//   10, 8
//   10, 9

You need to understand JS stack environment (I don't if it is the right term) to understand such a behavior.

In scheme for example You just can't produce such thing (Ok, ok -- with the help of underlying languages' references You can make it):

(define (make-ten-functions)
  (define (iter i)
    (cond ((> i 9) '())
          (else (cons (lambda () i) (iter (+ i 1))))))
  (iter 0))

(for-each (lambda (f)
            (display (f))
            (newline)) (make-ten-functions))

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