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JavaScript prototype inheritance and html canvas

I'm a Ruby developer who finally decided to learn JavaScript seriously. So I purchased some books and I started to dive in, but I got stuck quickly when I tried to understand prototypal inheritance...
One of the examples of the book is the following.
Given a Shape which prototype has a draw method, and two child shapes: a Triangle and a Rectangle which prototype inherit from Shape;
when I call the draw function on Triangle and Rectangle instances the method will draw them properly.
when I add a second method to show their name, every instance will log it properly.
Everything was understandable perfectly until I added a third method to fill the shapes... And only the last one get filled. no matter which one I call. Why? Is there something special in canvas?
Here is the code of the exercise:
function Point(x, y) {
this.x = x;
this.y = y;
}
function Shape() {
this.points = [];
this.init();
}
Shape.prototype = {
constructor: Shape,
init: function() {
if (this.context === undefined) {
Shape.prototype.context = document.getElementById('canvas').getContext('2d');
};
if (this.name === undefined) {
Shape.prototype.name = 'generic shape'
}
},
draw: function() {
var i, ctx = this.context;
ctx.strokeStyle = 'rgb(0,0,255)';
ctx.beginPath();
ctx.moveTo(this.points[0].x, this.points[0].y);
for (i = 1; i < this.points.length; i++) {
ctx.lineTo(this.points[i].x, this.points[i].y);
}
ctx.closePath();
ctx.stroke();
},
fill: function(color) {
var ctx = this.context;
ctx.fillStyle = color;
ctx.fill();
},
say_name: function() {
console.log('Hello my name is ' + this.name)
}
};
function Triangle(a, b, c) {
this.points = [a, b, c];
this.name = 'Triangle'
this.context = document.getElementById('canvas').getContext('2d');
}
function Rectangle(side_a, side_b) {
var p = new Point(200, 200);
this.points = [
p,
new Point(p.x + side_a, p.y), // top right
new Point(p.x + side_a, p.y + side_b), // bottom right
new Point(p.x, p.y + side_b) // bottom left
];
this.name = 'Rectangle'
this.context = document.getElementById('canvas').getContext('2d');
}
(function() {
var s = new Shape();
Triangle.prototype = s;
Rectangle.prototype = s;
})();
function testTriangle() {
var p1 = new Point(100, 100);
var p2 = new Point(300, 100);
var p3 = new Point(200, 0);
return new Triangle(p1, p2, p3);
}
function testRectangle() {
return new Rectangle(100, 100);
}
function make_me_crazy() {
var t = testTriangle();
var r = testRectangle();
t.draw();
r.draw();
t.say_name();
r.say_name();
t.fill('red');
}
make_me_crazy();
<canvas height='600' width='800' id='canvas' />
Thank you!
More details:
Why the function say_name is working exactly I expect saying: 'I am a triangle' or 'I am a rectangle' and never 'I am a generic shape', but the fill function fills the rectangle despite I'm calling it on a triangle instance? As people rightly answered to flip the two draw functions calls, I would specify better the following. The problem is not about the color of a shape, but the context pointer. why only the last shape is filled? If I add more shapes before calling fill only the last one get filled. This means I'm doing something wrong referring to the canvas. I supposed it was "the place where I draw shapes" but it seems more like "the last active shape"
How can I fix that code to make it working correctly filling the shape I want whenever I want? I mean. what if I want to have a function which receive an instance of a particular shape and fills it?
Is there any way to access a the draws contained into a canvas?

The core of the problem is the context - your shapes are sharing the single context of the canvas, and therefore it is not straight-forward to flip back and forth between objects. Instead, think of your order-of-operations as handling a single shape at a time and only moving on to the next one when you are done with the former.
Note the order of calls in the make_me_crazy function:
function Point(x, y) {
this.x = x;
this.y = y;
}
function Shape() {
this.points = [];
this.init();
}
Shape.prototype = {
constructor: Shape,
init: function(){
if (this.context === undefined) {
Shape.prototype.context = document.getElementById('canvas').getContext('2d');
};
if(this.name === undefined){
Shape.prototype.name = 'generic shape'
}
},
draw: function(){
var i, ctx = this.context;
ctx.strokeStyle = 'rgb(0,0,255)';
ctx.beginPath();
ctx.moveTo(this.points[0].x, this.points[0].y);
for (i = 1; i<this.points.length; i++) {
ctx.lineTo(this.points[i].x, this.points[i].y);
}
ctx.closePath();
ctx.stroke();
},
fill: function(color){
var ctx = this.context;
ctx.fillStyle = color;
ctx.fill();
},
say_name: function(){console.log('Hello my name is '+ this.name)}
};
function Triangle(a,b,c){
this.points = [a, b, c];
this.name = 'Triangle'
this.context = document.getElementById('canvas').getContext('2d');
}
function Rectangle(side_a, side_b){
var p = new Point(200, 200);
this.points = [
p,
new Point(p.x + side_a, p.y),// top right
new Point(p.x + side_a, p.y + side_b), // bottom right
new Point(p.x, p.y + side_b)// bottom left
];
this.name = 'Rectangle'
this.context = document.getElementById('canvas').getContext('2d');
}
(function(){
var s = new Shape();
Triangle.prototype = s;
Rectangle.prototype = s;
})();
function testTriangle(){
var p1 = new Point(100, 100);
var p2 = new Point(300, 100);
var p3 = new Point(200, 0);
return new Triangle(p1, p2, p3);
}
function testRectangle(){
return new Rectangle(100, 100);
}
function make_me_crazy(){
var t = testTriangle();
t.say_name();
t.draw();
t.fill('red');
var r = testRectangle();
r.draw();
r.say_name();
}
make_me_crazy();
<canvas height='600' width='800' id='canvas'></canvas>

About the points of your question.
For the first one: the key is this line of code
if(this.name === undefined){
Shape.prototype.name = 'generic shape'
}
When you instantiate Rectangle and Triangle, both of them set name.
In the other hand, the render method is only available in the Shape prototype.
About the second point (and the third one):
Maybe are you painting the Rectangle over the Triangle. Try to switch the order of the draw calls to check it.

Updating properties of JS "class" based on other properties?

I'm relatively new to Javascript and I am trying to create a very simple physics engine for a game type project I am working on. In order to do this, I created what I understand to be the JS equivalent of a class that I can create new copies of for each object I want. The problem is that I want to be able to update a value such as the x position and have this also update things such as the x Middle position (x center of object on screen). I know this is possible by using an object literal and the getter, however I want to be able to create new objects at realtime based on what's on the screen and I couldn't figure out how to use get to make this work. Here's the general idea of what I am trying to do:
var object = function (xPos, yPos, width, height) {
this.xPos = xPos;
this.yPos = yPos;
function getXMid (xP) { return xP + width/2; }
this.xMid = getXMid (this.xPos);
function getYMid (yP) { return yP + height/2; }
this.yMid = getYMid (this.yPos);
}
var ball = new object (10, 20, 50, 50);
ball.xPos = 50;
console.log (ball.xMid); // want this to output 75 instead of 45

You're changing one property, and expecting other properties to update, unfortunately it doesn't work that way when the properties hold primitive values.
You could use setters and getters and a function to update the other properties when you set a value
var object = function(xPos, yPos, width, height) {
this._xPos = xPos;
this._yPos = yPos;
this.recalc = function() {
this.xMid = getXMid(this.xPos);
this.yMid = getYMid(this.yPos);
}
Object.defineProperty(this, 'xPos', {
get: function() {
return this._xPos;
},
set: function(v) {
this._xPos = v;
this.recalc();
}
});
Object.defineProperty(this, 'yPos', {
get: function() {
return this._yPos;
},
set: function(v) {
this._yPos = v;
this.recalc();
}
});
function getXMid(xP) { return xP + width / 2; }
function getYMid(yP) { return yP + height / 2; }
this.recalc();
}
var ball = new object(10, 20, 50, 50);
ball.xPos = 50;
console.log (ball.xMid); // want this to output 75 instead of 45

Few Instance of object Javascript

I want to create a few instance of this class
var fruit = {
texture: new Image(),
speed: 5,
x: 0,
y: 0,
};
function fruits(speed, x, y)
{
fruit.speed = speed;
fruit.x = x;
fruit.y = y;
return fruit;
};
but when i create new object the all value was overridet by last created object. How can i repair this?
My loop:
var apples = [];
for(var i = 0; i < 10; i++)
{
apples[i] = new fruits(5, Math.floor((Math.random()*775)+1), 0);
apples[i].texture.src = "_img/apple.png";
}

The other answers which are appearing here are just bizarre. Here's the solution:
function fruits(speed, x, y)
{
this.texture = new Image( );
this.speed = speed;
this.x = x;
this.y = y;
};
Notice that the keyword this is used to set attributes. That means that when you call
var apple = new fruits( blah blah );
then apple will be set to a new object which has texture, speed, x and y attributes. There is no need to reference some global object to store these; they are stored in the newly created object itself.
Also I would rename it; the convention is to use singular names and a capital first letter for objects, so Fruit would make more sense (allowing new Fruit(...))

function Fruit( speed, x, y ){
var fruit = {}; // or use some base object instead of {}
fruit.texture = new Image();
fruit.speed = speed || 5;
fruit.x = x || 0;
fruit.y = y || 0;
return fruit;
};
var apples = [];
for( var i=0; i<10; i++ ){
apples[i] = Fruit( 5, Math.floor((Math.random()*775)+1), 0 );
apples[i].texture.src = "_img/apple.png";
}
Douglas Crockford - Power Constructor, 'new', 'this' and more

You got an object here:
var fruit = {
texture: new Image(),
speed: 5,
x: 0,
y: 0, // Note the superflous comma, which might break the code in some IE versions
};
And a function here:
function fruits(speed, x, y) {
fruit.speed = speed;
fruit.x = x;
fruit.y = y;
return fruit;
};
The function modifies above object whenever it is called and returns it.
Now, what you want is a constructor, but you don't have one here.
This, would be a constructor for a new Fruit:
function Fruit(speed, x, y) {
this.texture = new Image();
this.speed = speed || 5; // Note: Using logical OR to emulate default values for the argument
this.x = x || 0;
this.y = y || 0;
// Note: There is no return here!
}
var a = new Fruit(2, 1, 10);
var b = new Fruit(4, 10, 20);
a === b; // Returns false, you got two instances :)
new may have the functionality of being able to create instances of a Function, but you can still override this behavior by returning manually from within the constructor Function.
Also, even if you left out the return fruit in your original code, you would get back an empty instance of fruits since you don't assign any properties to the newly created instance.
In my Fruit example I reference the instance object via the this keyword, so I can assign speed, image, x and y to each instance created.
You might also want to read:
http://bonsaiden.github.io/JavaScript-Garden/#function.constructors
http://bonsaiden.github.io/JavaScript-Garden/#function.this

function fruits(speed, x, y) {
return {
texture: new Image(),
speed: speed,
x: x,
y: x,
}
};

Try such constructor:
function Fruit(speed, x, y) {
return {
speed: speed,
x: x,
y: y
}
}
alert(new Fruit("mySpeed", 1, 2).speed);

javascript setting attribute of prototype to attribute of object

To begin with, I don't fully understand the prototypal structure of Javascript so this may not be possible.
If I have ...
var vector = function( x, y ) {
this.x = x || 0;
this.y = y || 0;
}
var obj = function() {
this.position = new vector( 0, 0, 0 );
}
var cube = new obj();
... how can I add a property x to obj such that calling cube.x is equivalent to cube.position.x. I think it should be possible to make properties of each reference the same value, but I'm just not sure of the syntax. Something like obj.prototype.x = obj.position.x doesn't work because obj.position is undefined.
I would like the following behaviour to be possible
alert(cube.position.x); // 0
alert(cube.x); // 0
cube.position.x = 2;
alert(cube.position.x); // 2
alert(cube.x); // 2
cube.x = 4;
alert(cube.position.x); // 4
alert(cube.x); // 4
Is this possible?
I should probably mention that I'm working with Three.js so rewriting the objects isn't an option, just adding to them and their prototypes.

To get cube.x to return whatever cube.position.x contains, you'd need to define accessors and mutators for obj.prototype.x. Accessors and mutators are a relatively newer feature in JavaScript, and are not supported in most versions of IE.
var vector = function( x, y ) {
this.x = x || 0;
this.y = y || 0;
}
var obj = function() {
this.position = new vector( 0, 0, 0 );
}
obj.prototype = {
...your prorotype methods here...
};
Object.defineProperty(obj.prototype, 'x', {
get: function () {
return this.position.x;
},
set: function (val) {
this.position.x = val;
}
});
//alternatively:
obj.prototype = {
get x() {
return this.position.x;
},
set x(val) {
this.position.x = val;
}
};
var cube = new obj();
cube.x; //0
cube.x = 10;
cube.position.x; //10

Initiation of an object inside another

I have a piece of js software that is structured like so:
obj = new object[id]();
function wrapperFunction (e) {
var pos = findPos(this);
e._x = e.pageX - pos.x;
e._y = e.pageY - pos.y;
var func = obj[e.type];
if (func) {
func(e);
}
}
__
obj.line = function () {
this.started = false;
this.mousedown = function (e) {
}
this.mousemove = function (e) {
if (this.started) {
}
}
this.mouseup = function (e) {
if (this.started) {
}
}
}
The above code block is duplicated for multiple shapes so there is also a obj.square obj.circle etc...
I also have a shape object that is as follows.
function Shape (type, color, height, width, radius, x, y) {
this.type = type;
this.color = color;
this.h = height;
this.w = width;
this.r = radius;
this.points = ["x","y"];
this.points["x"] = [x];
this.points["y"] = [y];
};
I would like to initiate the shape object on a mousedown for each obj.* and populate the shape object with the propper info.
Now for the issue.
The radius is calcuated on every mousemove as well as height and width but when I add shapes = new Shape(circle, black, 10, 10, null, e._x, e._y) to the mousemove so it looks like...
this.mousemove = function (e) {
if (this.started) {
shapes = new Shape(circle, black, 10, 10, null, e._x, e._y);
}
}
The shape object does not create.
If I create the shape object inside the wrapper function instead of the mousemove then the object initiates but I cannot use radius or height/width.
How can I create an object inside another object inside a wrapper function so I can use calculated terms inside the created object? Is there an alternate route to take besides what I am doing?

Aside from wonkiness in the obj = new object[this.id](); line, I think you're just missing a this keyword:
this.mousemove = function (e) {
if (this.started) {
this.shapes = new Shape(circle, black, 10, 10, null, e._x, e._y);
}
}
Edit just noticed more wonkiness in your code (yes, that's a technical term :). I think you want to change these lines in the constructor:
this.points = ["x","y"]; // creates an array, which is indexed by numbers
this.points["x"] = [x]; // tacks on some ad-hoc properties to the array, which
this.points["y"] = [y]; // doesn't really make sense
to this:
this.points = {x: x, // I think this is what you actually mean to do.
y: y};