add simple rect

js/main.js

@@ -1,5 +1,16 @@
 window.addEventListener("load", () => main());
 
 const main = () => {
-    console.log("Hi!");
+    const cnv = document.getElementById("canvas");
+    const ctx = cnv.getContext("2d");
+
+    const size = 100;
+
+    ctx.fillStyle = "blue";
+    ctx.fillRect(
+        cnv.width / 2 - size / 2,
+        cnv.height / 2 - size / 2,
+        size,
+        size
+    );
 };