The arguments that that canvas arc takes for the start and stop angles should be in radians and not degrees. The concept of a radian is thinking of angles in terms of the value of pi times two rather than 360 degrees. If you prefer to think in depress you will still want to know how to convert from degrees to radians as well as the inversion of that.
To convert a degree value to a radian value to be used with the canvas arc method just divide the degree value by 180 and then multiply that value by Math.PI to get the equivalent value in radians.
The canvas arc method takes up to six arguments. The first two arguments given to the arc method set the center x and center y values of the arc. The third argument is the radius of the arc, and then the next two arguments after that is the beginning and ending angle in radians. The last optional argument is used to set clockwise of counter clockwise direction of the arc.
The canvas arc method can be used in conjunction with other line methods such as ctx.lineTo, more on that later. There is also the ctx.stroke, and ctx.fill methods and setting the style of lines and fills as well that you should be familiar with as well, but I do not want to get to far of base when it comes to drawing in general with canvas.
So the canvas arc method will come in handy for most situations when it comes to drawing arcs, and circles in canvas. However there is also using the Math.cos and Math.sin methods in combination with canvas methods like ctx.moveTo and ctx.lineTo as well. If for some reason you want better control over the drawing of arc like curves in canvas, and feel compelled to write your own solution for drawing arcs, or something to that effect you might want to play around with those methods a little as well.
To draw a full circle with the canvas arc method just set radian values from zero to Math.Pi * 2. This is a much quicker option to taking the time to write a polygon method to draw a circle, although writing that kind of method would give a greater deal of control over the various factors when it comes to daring a circle in canvas.
This works fine in most cases when I just want to quickly draw a circle. However there might be a desire in some cases to set other values such as the number of points in the arc and so forth. In that case I might need to take the time to write my own method for drawing an arc, or fine something else.
The sixth option argument can be used to set clockwise or counter clockwise direction of the drawing of the canvas arc. This can be used in conjunction with proper values for the start and end radian values to do things like drawing a shape that looks like an eaten watermelon slice for example.
Drawing both chords and just plain arcs can be achieved with the canvas arc method by just simply including or excluding the closePath context method. The close path method will draw the final line back to the starting point, while not calling it will not do that of course.
Another topic that comes to mind is the idea of wrapping the canvas arc method in a function and then setting some hard coed defaults for the method. For example in just about all use case examples of the canvas arc method I am using the method to draw a circle. So why not make a method where there are some defaults for arguments that make the starting radian value zero, and the ending radian value the value of pi times two. This way I do not have to do so each time I call the canvas arc method, and if I need to I can still pass different values for the starting and ending radian.
It is fun to write these kind of methods now and then to gain a better degree of control over how the arc, or circle is drawn. Many canvas libraries have a polygon method built in, but with plain vanilla js it is not to hard to start to get together some methods for drawing a polygon with a set number or points.
This method can only be used to draw a circle, rather than say a half circle as I have choses to omit arguments for a start and end radian, and direction. It is true that writing a clone of the canvas arc method would not to be to hard, but doing so would not make sense, unless there are some additional features to add, such as being able to set the number of sides in the canvas arc.
The canvas arc method is just one of many methods in the canvas 2d drawing context of course, however it is one that seems to come up often. I hope that you have gain something of value from reading this post, canvas is a lot of fun of course, and it can also be very helpful as well when it comes to working out basic graphics in canvas.