Explicação da Net

function drawShape(){

  function draw() {

      var canvas = document.getElementById("canvas");

      if (canvas.getContext) {

        var ctx = canvas.getContext("2d");



ctx.fillRect(25,25,100,100);

ctx.clearRect(45,45,60,60);

  ctx.strokeRect(50,50,50,50);   

      }

    }