Programming with Functions - Stuttgart JS Meetup

programmingWithFunctions.js

// ***************************
// Programming with functions  (Kamil Jopek)
// Stuttgart JS Meetup Talk (13th April 2015)
// ***************************

// Basic functions
var add = function(a, b) {
  return a + b;
};

var multiply = function(a, b) {
  return a * b;
};

var square = function(a) {
  return a * a;
};

// convert arguments object into real array
var array = function(args) {
  return Array.prototype.slice.call(args);
};

// ***************************
// Encapsulation with closure
// ***************************
var generateIterator = function(num) {
  var i = num || 0;
  return function() {
    return i++;
  };
};

var iterator = generateIterator(6);

// ***************************
// Map/Reduce
// ***************************
var map = function(fn, array) {
  var result = [];
  for (var i = 0; i < array.length; i++) {
    result[i] = fn.call(this, array[i]);
  }
  return result;
};

var reduce = function(fn, memo, array) {
  for (var i = 0; i < array.length; i++) {
    memo = fn.call(this, array[i], memo);
  }
  return memo;
};

// ***************************
// Compose
// ***************************
var composeNaive = function(f, g) {
  return function(value) {
    return f(g(value));
  };
};

var compose = function(/*functions*/) {
  var functions = array(arguments).reverse();
  return function(value) {
    for (var i = 0; i < functions.length; i++) {
      value = functions[i].call(this, value);
    }
    return value;
  };
};

// ***************************
// Reverse and pipeline
// ***************************

var reverse = function(fn) {
  return function(/*arguments*/) {
    var args = array(arguments).reverse();
    return fn.apply(this, args);
  };
};

var pipeline = reverse(compose);

// ***************************
// Partial application
// ***************************
var partial = function(/*function, arguments*/) {
  var originalArgs = array(arguments),
      fn = originalArgs.shift();
  
  return function(/*arguments*/) {
    return fn.apply(this, originalArgs.concat(array(arguments)));
  };
};

// ***************************
// Curry
// ***************************
var curry = function(fn) {  
  function curried(args) {
    if (args.length >= fn.length) {
      return fn.apply(this, args);
    } else {
      return function() {
        return curried(args.concat(array(arguments)));
      }
    }
  };  
  
  return curried([]);
};

// basic example - not real life use case
var addFiveNumbers = curry(function(a, b, c, d, e) {
  return a + b+ c + d + e; 
});

addFiveNumbers(1, 2, 3)(45)(5);

// map reduce with curry - real world use case

map = curry(map);
reduce = curry(reduce);

var people = [
  {name: 'Jack', age: 24, cars: 2},
  {name: 'Jane', age: 19, cars: 1}
];

var get = curry(function(attribute, object) {
  return object[attribute];
});


// create functions with one argument
var mapToCars = map(get('cars'))
var sum = reduce(add, 0);

// compose them together
var sumAllCars = compose(sum, mapToCars);

console.log(sumAllCars(people));