functional-programming-week-01.md

Functional Javascript

Soul of Functional Programming: Separation and Recognition

• Anything goes
• Everything changes
• Weird names

The symptoms

• custom names
• looping patterns
• glue code
• side effects

omit needless names

• Separate inputs from environment

Secret Input: time (Hard to test)

function daysThisMonth() {
	var date = new Date(),
		y = date.getFullYear(),
		m = date.getMonth(),
		start = new Date(y, m, 1),
		end = new Date(y, m + 1, 1);
	return (end - start) / (1000 * 60 * 60 * 24);
}

Always works the same

function daysInMonth(y, m) {
	var start = new Date(y, m - 1, 1),
		end = new Date(y, m, 1);
	
	return (end - start) / (1000 * 60 * 60 * 24);
}

Sperating Mutation from Calculation

Isolate the place where the mutation happen.

Teaser updates DOM

function teaser(size, elt) {
	setText(elt, slice(0, size, text(elt)));
}

map(teaser(50), all('p'));

Merely calculates

var teaser = slice(0);
map(compose(setText, teaser(50), text), all('p'));

recognize pure functions

Functions that don't change anything are called "pure".

• testable
• portable
• memoizable (techinique once you run a function it will memoize and the second time it will return the value and not execute the function)
• parallelizable

function random(m_w, m_z) {
	m_z = 36969 * (m_z & 65535) + (m_z >> 16);
	m_w = 18000 * (m_w & 65535) + (m_w >> 16);
	return (m_z << 16) + m_w;
}

separete function from Rules

functions are nouns. We define functions by rules, but think it by nouns. Every function is a single-valued collection of pairs.

separete arity form functions

function get(property, object) {
	return object[property];
}

var people = [{name}]

Args up front

function getPersonName(person) {
	return get('name', person);
}

var names = people.map(getPersonName)

More args later

// MAGIC!

var names = people.map(get('name'));

Currying: put things one at time.

function curry(fn) {
	return function () {
		if (fn.length > arguments.length) {
			var slice = Array.prototype.slice;
			var args = slice.apply(arguments);
			return function () {
				return fn.apply(
					null, args.concat(slice.apply(arguments)));
				)
			};
		}
		return fn.apply(null, arguments);
	};
}

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

var names = people.map(get('name'))