In Ruby, methods like map, reject, reduce are very handy to use. Recently, I came across Javascript functional programming, I found there are similar usages in Javascript.

map

map is used to transform an array into another

Here’s a simple example in Ruby, it accepts a block as parameter:

arr = ['m', 'o', 'n', 'k', 'e', 'y']
arr.map {|x| x + '1'}
 #=> ["m1", "o1", "n1", "k1", "e1", "y1"]

In Javascript, map accepts another function as a parameter, here is an example:

var arr = ['m', 'o', 'n', 'k', 'e', 'y'];
var arr2 = arr.map(function(x) {
  return x + '1';
});
console.log(arr2); 
// [ 'm1', 'o1', 'n1', 'k1', 'e1', 'y1' ]

In ES6, you can also do this with arrow function, which is much similar to Ruby way:

var arr = ['m', 'o', 'n', 'k', 'e', 'y'];
var arr2 = arr.map((x) => x + '1');
console.log(arr2);
// [ 'm1', 'o1', 'n1', 'k1', 'e1', 'y1' ]

reduce

reduce comes very handy to apply an operator on enumerable elements

Here is an example of reduce in Ruby, it takes an initial value and a block applying operation on element

arr = [1, 2, 3, 4, 5]
arr.reduce(0) {|sum, x| sum += x}
 #=> 15

reduce can also accept a symbol as a parameter, initial value is default set to 0 for :+, 1 for :*

arr = [1, 2, 3, 4, 5]
arr.reduce(:+)
 #=> 15

In Javascript, you can do this, the first parameter is a function that applies the operator, return value will be the next parameter that this function accepts, the second parameter is initial value:

var arr = [1, 2, 3, 4, 5];
var arr2 = arr.reduce(function(sum, x) {
  return sum + x;
}, 0);
console.log(arr2);
// 15

In ES6, you can do this, also I prefer this way as a Ruby programmer:

var arr = [1, 2, 3, 4, 5];
var arr2 = arr.reduce((sum, x) => sum + x, 0);
console.log(arr2);
// 15

＼（＾∀＾）メ（＾∀＾）ノ

Let’s have more fun in functional programming