javascript canvas -javascript design I am doing some tests with html5 Canvas object. At the same time I am trying to make my javascript code more modular.  In the following code, where/how should I declare canvas and context, so that it can be shared with other objects?  Please provide general comments/improvements?

Output:



Code:

$( document ).ready(function() {

    var axis = {
        LEFT_INDENT : 100,  
        START_WORK_HOUR : 8,
        END_WORK_HOUR : 17,
        HOURS_IN_WORKDAY : 0,
        CANVAS_WIDTH : 0,
        CANVAS_HEIGHT : 0,
        AXIS_SIDE_MARGIN : 20,
        AXIS_TOP_MARGIN : 40,
        AXIS_ORIGIN : new Array(),  
        AXIS_RIGHT : 0,
        TICK_INCREMENT :  0,

        setup: function(){

            // assumes that I have &lt;canvas id="myCanvas" width="800" height="300"&gt;&lt;/canvas&gt;  in the HTML
            this.canvas = document.getElementById('myCanvas');
            this.context = this.canvas.getContext('2d');

            this.HOURS_IN_WORKDAY = this.END_WORK_HOUR - this.START_WORK_HOUR;
            this.CANVAS_WIDTH = this.canvas.width;
            this.CANVAS_HEIGHT = this.canvas.height;

            this.AXIS_ORIGIN = { x: this.AXIS_SIDE_MARGIN,
                            y: this.AXIS_TOP_MARGIN };

            this.AXIS_RIGHT = this.canvas.width-this.AXIS_SIDE_MARGIN;

            this.TICK_INCREMENT = (this.CANVAS_WIDTH-this.LEFT_INDENT-(2*this.AXIS_SIDE_MARGIN))/this.HOURS_IN_WORKDAY; 
            this.AXIS_RIGHT = this.canvas.width-this.AXIS_SIDE_MARGIN;
        },

        draw_lines: function(){
            this.context.beginPath();
            this.context.moveTo(this.LEFT_INDENT, this.AXIS_ORIGIN.y);
            this.context.lineTo(this.AXIS_RIGHT,  this.AXIS_ORIGIN.y)
            this.context.stroke();
            this.counter = this.START_WORK_HOUR;

            for(x=this.AXIS_SIDE_MARGIN+this.LEFT_INDENT ; x &lt; this.AXIS_RIGHT ; x = x + this.TICK_INCREMENT){
                // grid lines
                this.context.beginPath();
                this.context.moveTo(x,30);
                this.context.lineTo(x,50)
                this.context.stroke();

                // background line - top to bottom
                this.context.save();   
                this.context.fillStyle = "#F0F8FF";
                this.context.beginPath();
                this.context.lineWidth = .1;
                this.context.moveTo(x,30);
                this.context.lineTo(x,this.CANVAS_HEIGHT-20)
                this.context.stroke();
                this.context.restore();   

                // text     
                this.context.font = "14px Arial";
                this.context.fillStyle = "#011f5b";     
                this.counter++;
                this.context.fillText(this.counter+":00",x-7,20);       
            }   
        }
    }
    axis.setup();
    axis.draw_lines();
});
You can wrap the functionality in an object. You instantiate it with a canvas element and some configuration options.

(function($) {

// config defaults    
var defaults = {
    LEFT_INDENT: 100,
    START_WORK_HOUR: 8,
    END_WORK_HOUR: 17,
    AXIS_SIDE_MARGIN: 20,
    AXIS_TOP_MARGIN: 40
};

// constructor
axis = function(canvas, config)
{
    this.config = $.extend({}, defaults, config);
    this.canvas = canvas;

    this.setup();
}

// define prototype functions
axis.prototype.setup = function() {
    this.context = this.canvas.getContext('2d');

    this.HOURS_IN_WORKDAY = this.config.END_WORK_HOUR - this.config.START_WORK_HOUR;
    this.CANVAS_WIDTH = this.canvas.width;
    this.CANVAS_HEIGHT = this.canvas.height;

    this.AXIS_ORIGIN = {
        x: this.config.AXIS_SIDE_MARGIN,
        y: this.config.AXIS_TOP_MARGIN
    };

    this.AXIS_RIGHT = this.canvas.width - this.config.AXIS_SIDE_MARGIN;

    this.TICK_INCREMENT = (this.CANVAS_WIDTH - this.config.LEFT_INDENT - 2*this.config.AXIS_SIDE_MARGIN)) / this.HOURS_IN_WORKDAY;
}

axis.prototype.draw_lines = function() {
    this.context.beginPath();
    this.context.moveTo(this.LEFT_INDENT, this.AXIS_ORIGIN.y);
    this.context.lineTo(this.AXIS_RIGHT,  this.AXIS_ORIGIN.y)
    this.context.stroke();
    this.counter = this.config.START_WORK_HOUR;

    for(x = this.config.AXIS_SIDE_MARGIN + this.config.LEFT_INDENT ; x &lt; this.AXIS_RIGHT ; x = x + this.TICK_INCREMENT) {
        // grid lines
        this.context.beginPath();
        this.context.moveTo(x,30);
        this.context.lineTo(x,50)
        this.context.stroke();

        // background line - top to bottom
        this.context.save();   
        this.context.fillStyle = "#F0F8FF";
        this.context.beginPath();
        this.context.lineWidth = .1;
        this.context.moveTo(x,30);
        this.context.lineTo(x, this.CANVAS_HEIGHT - 20)
        this.context.stroke();
        this.context.restore();   

        // text     
        this.context.font = "14px Arial";
        this.context.fillStyle = "#011f5b";     
        this.counter++;
        this.context.fillText(this.counter+":00",x-7,20);       
    }   
}

}(jQuery));


To use it:

var a = new axis(document.getElementById('myCanvas'), {
     // whatever you want to override
});
a.draw_lines();