The arc method can be used when drawing a line by using the beginPath method at which point the arc method can be used in conjunction with other method like moveTo lineTo and so forth to help draw shapes.
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.
To draw a full circle with the arc method just set radian values from zero to Math.Pi * 2.
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.
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.