Functional Programming with JavaScript

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Functional
Programming
with JavaScript

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Agenda
1. What
a. are functions?
b. is functional programming?
2. How?
3. Why?
4. Cool Stuff

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Ramda lodash-fp

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Functions
A relation f from a set A to a set B is
said to be a function if every element
of set A has one and only one image
in set B.

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Functional Programming is about using
functions as units of abstraction and
composing them to build a system.

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Functional language or a language
facilitating functional programming is one
which supports first-class functions.

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R.map(function (n) {
return addOne(n);
}, list);

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R.map(function (n) {
return addOne(n);
}, list);
R.map(addOne, list);

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$http.get('/list', function(res) {
callback(res);
});

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$http.get('/list', function(res) {
callback(res);
});
$http.get('/list', callback);

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Example: Searching a list
Declarative
var nameEquals = R.compose(
R.match(new RegExp(value, 'i')), R.prop('name')
);
var filtered = R.filter(nameEquals, list);
Imperative
var filtered = [], i = 0, pattern = new RegExp(value, 'i');
for (i = 0; i < list.length; i += 1) {
if (list[i].name.match(pattern)) filtered.push(list[i]);
}

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Higher-order functions
are first-class functions, that can take as
argument or return a function.

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Map, Reduce and Filter
are the three most important functions to
work with collections.
Do not use any of these as a loop.

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If the input and output collections are always
going to be same length, you probably need map.
var double = (x) => x * 2;
R.map(double, [1, 2, 3]); //=> [2, 4, 6]

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If the output collection can have lesser or equal
number of items, you probably need filter.
var isEven = (n) => n % 2 === 0;
R.filter(isEven, [1, 2, 3, 4]); //=> [2, 4]

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You probably need reduce when the input is a
collection and the output is a single value.
Reduce is also known as fold.
R.reduce(R.add, 0, [1, 2, 3]); //=> 6

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Map, Reduce and Filter are low-level. There
are many others provided by libraries, which
can be a better fit.

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Example: Given an array of objects, get only the
‘name’ property values in an array.
R.map(R.prop('a'), [{a: 1}, {a: 2}]);
// pluck :: k -> [{k: v}] -> v
R.pluck('a', [{a: 1}, {a: 2}]); //=> [1, 2]

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1. Functions should be simple.
2. They should do and focus on only one thing.
3. Try writing small functions.
4. Do not mix concerns
Ref: Simple Made Easy by Rich Hickey

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Create lot of functions
Don’t be afraid to create many small functions.
Abstract whenever two functions seems to do similar
things or some part of them are similar.

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Open/closed principle
states "software entities (classes, modules,
functions, etc.) should be open for extension,
but closed for modification"
Ref: https://en.wikipedia.org/wiki/Open/closed_principle

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Function Composition
var x' = f(x) ;
var x'' = g(x'); } g(f(x));
var h = R.compose(g, f); h(x);

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Example
// notDone :: Object -> Boolean
var notDone = function (todo) {
return !todo.done;
}
var notDone =
R.compose(R.not, R.prop('done'));

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// wordCount :: String -> Number
var wordCount = function (s) {
return s.split(' ').length;
}
var wordCount = R.compose(
R.length, R.split(' ')
);
wordCount('How many words?') //=> 3

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Pass and return functions
If some function accepts many parameters to
perform some part of its operations try to
replace with function instead.

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Example: Pass function
function createBuiltFile (packageName, minifier, extension, files) {
...
var name = packageName + '.' + md5(content) + '.' + extension;
...
}
function createBuiltFile (minifier, nameGenerator, files) {
...
var name = nameGenerator(content);
...
}

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Pass functions instead of values
when you want to make a function more
generic.

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Work with simple data structures such
as Arrays and Objects

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Curried Functions
A function that returns a new function until it
receives all its arguments.
Originated by
Moses Schönfinkel
Developed by
Haskell Curry

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// add :: Number -> Number -> Number
var add = R.curry(function (x, y) {
return x + y;
});
// addOne :: Number -> Number
var addOne = add(1); addOne(5); //=> 6
A curried function can be easily specialized for
your own needs.

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fetchFromServer()
.then(JSON.parse)
.then(function(data){ return data.posts })
.then(function(posts){
return posts.map(function(post){
return post.title;
});
})
fetchFromServer()
.then(JSON.parse)
.then(R.prop('posts'))
.then(R.map(R.prop('title')))
Read: Why Curry Helps?

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Partial Application
// propEq :: k -> v -> {k: v} -> Boolean
var idEqZero = R.propEq('id', 0);
in this example we say that R.propEq has been
partially applied.

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● Helps you build new functions from old
one.
● Think of it as configuring a function for
your needs.
● It also makes composition easier.

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● Declarative
● Easy to reason about
● Easy to test
Why Functional Programming Matters (by John Hughes)

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Functional null checking
if (x !== null && x !== undefined) {
f(x);
}

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f(x);
}
Maybe(x).map(f); //=> Maybe(f(x))

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f(x);
}
R.map(f, Maybe(x)); //=> Maybe(f(x))

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f(x);
}
R.map(f, Maybe(x)); //=> Maybe(f(x))
var g = R.compose(R.map(f), Maybe);
g(x); //=> Maybe(f(x)) only if x != null

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f(x);
}
R.map(addOne, Maybe(2)); //=> Maybe(3)
R.map(addOne, [2]) //=> [3]

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Both Maybe and Array are Functors.

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Functor is something that implements map.
Some Functors are Array, Maybe, Either,
Future and many others.

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Fantasy Land Specification
https://github.com/fantasyland/fantasy-land

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bilby.js Folktale
ramda-fantasy

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● Start writing more functions
● Preferably pure and simple functions
● Glue your functions using higher-order
functions
● Use a functional library
● Discover!

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References
Books
● http://functionaljavascript.com/
● https://leanpub.com/javascriptallongesix/
● https://github.com/DrBoolean/mostly-adequate-guide
Talks
● https://www.youtube.com/watch?v=AvgwKjTPMmM
● http://scott.sauyet.com/Javascript/Talk/2014/01/FuncProgTalk/
● https://www.youtube.com/watch?v=m3svKOdZijA

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Thank You!
Debjit Biswas
@debjitbis08
https://in.linkedin.com/in/debjitbis08

Functional Programming with JavaScript

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