polygons.js

// Define a regular polygon with n sides, centered at (x,y) with radius r.
// The vertices are equally spaced along the circumference of a circle.
// Put the first vertex straight up or at the specified angle.
// Rotate clockwise, unless the last argument is true.
function polygon(c, n, x, y, r, angle=0, counterclockwise=false) {
    c.moveTo(x + r*Math.sin(angle),  // Begin a new subpath at the first vertex
             y - r*Math.cos(angle)); // Use trigonometry to compute position
    let delta = 2*Math.PI/n;         // Angular distance between vertices
    for(let i = 1; i < n; i++) {     // For each of the remaining vertices
        angle += counterclockwise?-delta:delta; // Adjust angle
        c.lineTo(x + r*Math.sin(angle),         // Add line to next vertex
                 y - r*Math.cos(angle));
    }
    c.closePath();                   // Connect last vertex back to the first
}

// Assume there is just one canvas, and get its context object to draw with.
let c = document.querySelector("canvas").getContext("2d");

// Start a new path and add polygon subpaths
c.beginPath();
polygon(c, 3, 50, 70, 50);                   // Triangle
polygon(c, 4, 150, 60, 50, Math.PI/4);       // Square
polygon(c, 5, 255, 55, 50);                  // Pentagon
polygon(c, 6, 365, 53, 50, Math.PI/6);       // Hexagon
polygon(c, 4, 365, 53, 20, Math.PI/4, true); // Small square inside the hexagon

// Set some properties that control how the graphics will look
c.fillStyle = "#ccc";    // Light gray interiors
c.strokeStyle = "#008";  // outlined with dark blue lines
c.lineWidth = 5;         // five pixels wide.

// Now draw all the polygons (each in its own subpath) with these calls
c.fill();                // Fill the shapes
c.stroke();              // And stroke their outlines