map-function.js

// Let's say we have an array of candy
var all_candy = [
	{
		name: 'Mars',
		price: 1,
		category: 'chocolate'
	},
	{
		name: 'Snickers',
		price: 1.1,
		category: 'Chocolate'
	},
	{
		name: 'Bounty',
		price: 1,
		category: 'chocolate'
	},
	{
		name: 'Leo',
		price: .8,
		category: 'Waffer'
	},
	{
		name: 'Ole',
		price: .4,
		category: 'Waffer',
		fake: true
	},
	{
		name: 'VegiBlah',
		price: 3.4,
		category: 'Misc',
		fake: false
	},

];
/* yes that's valid javascript, the last objects in the array are
a bit different, it's one of the reason javascript is terrible, because that sort of stuff is so common and possible
*/

// let's say we are cheap and we want a cheap_candy array (price<1
// for that we would need to loop trough all elements of the array, right?
var cheap_candy = [];
for (var i = 0; i < all_candy.length; i++) {
	if (all_candy[i].price < 1) cheap_candy.push(all_candy[i]);
}

// or we use filter
var cheap_candy = candy.filter(function(candy){
	return candy.price < 1;
});

// since ES6 they added a shorthand, so this works too:
var cheap_candy = candy.filter(c => c.price < 1);

/* 
consult the docs about filter:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter
*/
/*
there are a lot of different array iteration methods such as:
forEach, entries, every, some, filter, find, findIndex, keys,
map, reduce, reduceRight, values

The most important one is reduce, because reduce can be made into doing the
safe stuff as others, for instance to do the filter bit of candies with reduce:
*/
var cheap_candy = all_candy.reduce((previous, current) => {
  if (current.price < 1) previous.push(current)
  return previous;
, []);