00-02-03-functions2-factories.md

layout	title	class	date
post	Functions as Objects and Factories	fundamentals	2015-08-18 17:00:00 -0700

Functions are Objects too!

Begin with this function definition in the exercises below:

function paint(obj) {
	obj.color = 'red';
}

All functions are also objects, which has the following implications:

1. Functions can have aliases.

• Make an alias for paint and call that alias as a function.

2. Functions can be methods of objects.

• Convert paint into a method, so that it always paints its owner.

3. Functions can be arguments.

• Use paint as a callback with forEach on some array of objects.

• Use an anonymous function which paints things 'blue' as a callback to forEach.

5. Functions can have properties of their own.

• Use the length property to determine how many arguments function paint requires.

• Rewrite paint to use, instead of 'red', a property of itself called color. Then set that property to 'green'. Then paint something green.

6. Functions can have methods of their own.

• Give function paint a method which sets the color it uses to paint things. You should be able to use it like this:

paint.useColor('turquoise');
paint(obj); // --> sets obj's color to 'turquoise'

call and apply

• Use this paint function to change of color of both car and fence:

function paint(color) {
	this.color = color;
}
var car = {wheels:4}, fence = {length:20};

• Use Math.max to find the largest number in this array:

var nums = [9,5,8,2,4,11,8,2,7,1,0,99,25,3,6,42];

this pitfalls

There's a problem in the code below:

function talk() {
	console.log(this.noise);
}

var animals = [dog, cat, canary];
animals.allTalk = function() {
	this.forEach(talk);
}
animals.allTalk(); // failure!

Identify the problem and fix it! Rewrite the allTalk method of array animals to
make each animal talk. You must use function talk without changing its definition, and forEach(something). something should be a function which is not talk but instead somehow calls talk.

Instances and Factory functions

We know how to make animal instances by hand:

var dog = {
	name:'dog',
	noise:'woof',
	talk:function() {
		console.log(this.noise);
	}
}
var sheep = {
	name:'sheep',
	noise:'baaa',
	talk:function() {
		console.log(this.noise);
	}
}

Write a function to create any number of animals like those above.

Sharing Instance Methods

Write two versions of a factory which makes animal instances which all share a single copy of their talk method.

1. First implement talk as a global function.

2. Instead of using a global function, attach talk as a property of the factory itself, but expect it to be called through individual animals.

Finding one's marbles

1. Write a marble factory which can generate marbles with a custom size and color (having a property for each).

2. Give each marble instance a method isBigger(other) which returns true if it is bigger than another marble, other. Attach that method to the factory initially, then link each marble instance to it, so that they all share the same function.

In the bag

Next, you'll implement an abstract data structure for a bag, which contains multiple items (e.g. marbles) jumbled together.

1. Write a factory function makeBag(items) which makes bag instances. Each bag holds a collection of items, and the parameter items is an array listing the items the bag starts with (though more may be added). Each bag instance should have the following methods:

   1. put(item): places item into the bag in an unspecified position.

   2. draw(): removes a random item from the bag and returns it.

   3. divide(criterionFn): removes from the bag all items for which the callback criterionFn(item) returns truthy, and returns a new bag with just those items. The original bag will retain the items not matching criterionFn.

   4. serialize(compareFn): returns an array containing all items in the bag, sorted according to compareFn.

2. Make a bag and fill it with 100 marbles of random size 1-4, with (nearly) equal numbers of red, blue, and green.

   1. Use your divide method to extract the red marbles into another bag.

   2. serialize the red marbles into an array sorted by marble size, large to small.

   3. serialize the non-red marbles from the original bag, sorted by color (blues before greens).

3. Make another bag which holds the names of everyone in the class. Serialize it in reverse alphabetical order.