问无法清除HTML5中的画布

EN

window.onload = function(){
    init();
    drawAll();  
}
function clear(){
    ctx2.clearRect(0,0,WIDTH,HEIGHT);
}

function init() {
    city.src ="city.png";
    image2.src="image.png";
    layer1 = document.getElementById("layer1");
    ctx1 = layer1.getContext("2d");
    layer2 = document.getElementById("layer2");
    ctx2 = layer2.getContext("2d");
}


function drawAll() {
    draw1();
    draw2();
}

function draw2() {
    ctx2.clearRect(0,0,WIDTH,HEIGHT);
    ctx2.drawImage(image2, 240, 200);
}

function draw1() {
    ctx1.clearRect(0, 0, WIDTH, HEIGHT);
    ctx1.drawImage(city, 0, 0);
}