var canvas = document.getElementById('canvas'),

    context = canvas.getContext('2d');

context.lineWidth = 30;

context.font = '24px Helvetica';

context.fillText('Click anywhere to erase', 175, 40);

context.strokeRect(75, 100, 200, 200);

context.fillRect(325, 100, 200, 200);

// 鼠标按下时清除整个画布

context.canvas.onmousedown = function (e) {

   context.clearRect(0, 0, canvas.width, canvas.height);

};

var canvas = document.getElementById('canvas'),

    context = canvas.getContext('2d'),

    gradient = context.createRadialGradient(

                 canvas.width/2, canvas.height, 10,

                 canvas.width/2, 0, 100);

gradient.addColorStop(0, 'blue');

gradient.addColorStop(0.25, 'white');

gradient.addColorStop(0.5, 'purple');

gradient.addColorStop(0.75, 'red');

gradient.addColorStop(1, 'yellow');

context.fillStyle = gradient;

context.rect(0, 0, canvas.width, canvas.height);

context.fill();

function drawText() {

context.fillStyle = 'blue';

context.strokeStyle = 'yellow';

context.fillText(text, canvas.width/2, canvas.height/2);

context.strokeText(text, canvas.width/2, canvas.height/2);

}

context.textAlign = 'center';

context.textBaseline = 'middle';

var canvas = document.getElementById('canvas'),

context = canvas.getContext('2d'),

image = new Image();

image.src = 'fence.png';

image.onload = function(e) {

    context.drawImage(image, 0, 0);

};

var canvas = document.getElementById('canvas'),

context = canvas.getContext('2d'),

scaleWidth = ..., // Calculate scales for width and height

scaleHeight = ...;

...

context.drawImage(canvas, 0, 0, scaleWidth, scaleHeight);