functional.js

// Built-in Higher Order Functions for Array:
//   map, filter, forEach, some, every, reduce

// The Emperor’s New Closure: FP in Javascript
// https://www.youtube.com/watch?v=a6azjixVy2g

l = [1,2,3,4,5]
function is_even(n) { return n%2==0; };         // predicate
function larger_than(n) { return function(m) { return m > n; } };
l.reduce(function (a,b) { return a+b; });       // sum(l)
l.filter(is_even);                              // [2,4]
l.every(function (n) { return n<6; });          // true
l.every(is_even);                               // false
l.filter(is_even).every(is_even)                // true
l.some(is_even);                                // true
l.some(larger_than(4));                         // true
l.some(larger_than(5));                         // false

l.map(Math.sqrt)	// [sqrt(1), sqrt(2), sqrt(3), sqrt(4), sqrt(5)]

l = '123456789'.split('')	// [ '1', '2', '3', '4', '5', ...]
Array.prototype.sum = function() { return this.reduce(function (a,b) { return a+b; }); }
l.sum();			// '123456789'
l.sum (l.map(Number));		// 45