map filter reduce

tmp.js

var arr1 = [1, 2, 3, 4, 5];
var arr2 = ["one", "two", "three", "four", "five"];

map
[a] -> (a -> b) -> [b]

function double (v) { return v + v }

var a = arr1.map(double)
// a === [2, 4, 6, 8, 10]

var b = arr1
  .map(double)
  .map(double)
// b === [4, 8, 12, 16, 20]

var e = arr2
 .map(function (v) { return v.length })
// e === [3, 3, 5, 4, 4]


filter
[a] -> (a -> Boolean) -> [a]

function greaterThan2 (v) { return v > 2 }

var c = arr1.filter(greaterThan2) // [3, 4, 5]

var d = arr1
  .filter(greaterThan2)
  .map(double)
// d === [6, 8, 10]

reduce
[a] -> (b -> a -> b) -> b -> b

var f = arr1.reduce(function (value, v) { return value + v}, 0)
// f === 15
value | v |
0     | 1 | 1
1     | 2 | 3
3     | 3 | 6
6     | 4 | 10
10    | 5 | 15

function mapper (fn) {
  return function _mapper (acc, v) {
    acc.push(fn(v))
    return acc;
  }
}

var doubler = mapper(double)
// function _mapper (acc, v) {
//   acc.push(double(v));
//   return acc;
// }

var g = arr1.reduce(doubler, [])

value        | v | eventual return
[]           | 1 | [2]
[2]          | 2 | [2, 4] // [2, double(2)]
[2, 4]       | 3 | [2, 4, 6] // [2, 4, double(3)]
[2, 4, 6]    | 4 | [2, 4, 6, 8] // [2, 4, 6, double(4)]
[2, 4, 6, 8] | 5 | [2, 4, 6, 8, 10] // [2, 4, 6, 8, double(5)]
// [2, 4, 6, 8, 10]

function filterer (pred) {
  return function _filterer (acc, v) {
    if (pred(v)) acc.push(v)
    return acc;
  }
}

var checker = filterer(greaterThan2);
// function _filterer (acc, v) {
//   if (greaterThan2(v)) acc.push(v);
//   return acc;
// }

var h = arr1.reduce(checker, []);

value  | v | return
[]     | 1 | []
[]     | 2 | []
[]     | 3 | [3]
[3]    | 4 | [3, 4]
[3, 4] | 5 | [3, 4, 5]
// [3, 4, 5]