在JavaScript中编码乒乓球时没有显示

Question

我的代码给出了一个黑匣子输出为600*800尺寸，没有其他显示。

我尝试编码基本的pong游戏匿名函数定期调用update((函数，该函数将reset((相应地调用

<canvas id = "gc" width = "640" height = "480"> </canvas>
<script>
userPlankYPosition =40;
computerPlankYPosition = 40;
plankWidth = 10;
plankHeight =100;
ballPositionX=ballPositionY=50;
ballDimension =6;
XVelocity=YVelocity=4;
playerScore=ComputerScore=0;
computerPlankSpeedinY=4;
var canvas,canvasContext;
    window.onload=function() {
        canvas=document.getElementById('gc');
        canvasContext=canvas.getContext('2d');
        setInterval(update, 1000/30);
    canvas.addEventListener('mousemove',function(e){
        userPlankYPosition = e.clientY-plankHeight/2;
    });
};
function reset(){
    ballPositionX=canvas.width/2;
    ballPositionY=canvas.height/2;
    XVelocity-=XVelocity;
    YVelocity=3;
};
function update(){
    canvasContext.fillstyle='black';
    canvasContext.fillRect(0,0,canvas.width,canvas.height);
    canvasContext.fillstyle='white';
    canvasContext.fillRect(0,userPlankYPosition,plankWidth,plankHeight);
    canvasContext.fillRect(canvas.width-plankWidth,computerPlankYPosition,plankWidth,plankHeight);
    canvasContext.fillRect(ballPositionX-ballDimension/2,ballPositionY-ballDimension/2,ballDimension,ballDimension);
    canvasContext.fillText(playerScore,100,100);
    canvasContext.fillText(ComputerScore,canvas.width-100,100); 
    ballPositionX+=XVelocity;
    ballPositionY+=YVelocity;
    if(ballPositionY<0 && YVelocity <0){
        YVelocity-=YVelocity;
    }
    if(ballPositionY>canvas.height && YVelocity <0){
        YVelocity-=YVelocity;
    }
    if(ballPositionX<0 )
    {
        if(ballPositionY>userPlankYPosition && ballPositionY<userPlankYPosition+plankHeight){
            XVelocity-=XVelocity;
            dy=ballPositionY-(userPlankYPosition+plankHeight/2);
            YVelocity = dy*0.3;
        }
        else{
            ComputerScore++;
            reset();
        }
    }
    if(ballPositionX>canvas.width )
    {
        if(ballPositionY>computerPlankYPosition && ballPositionY<computerPlankYPosition+plankHeight){
            XVelocity-=XVelocity;
            dy=ballPositionY-(computerPlankYPosition+plankHeight/2);
            YVelocity = dy*0.3;
        }
        else{
            playerScore++;
            reset();
        }
    };
    if(computerPlankYPosition+plankHeight/2<ballPositionY){
        computerPlankYPosition+=computerPlankSpeedinY;
    }
    else{
        computerPlankYPosition-=computerPlankSpeedinY;
    }
    // canvasContext.fillstyle='black';
    // canvasContext.fillRect(0,0,canvas.width,canvas.height);
    // canvasContext.fillstyle='white';
    // canvasContext.fillRect(0,userPlankYPosition,plankWidth,plankHeight);
    // canvasContext.fillRect(canvas.width-plankWidth,computerPlankYPosition,plankWidth,plankHeight);
    // canvasContext.fillRect(ballPositionX-ballDimension/2,ballPositionY-ballDimension/2,ballDimension,ballDimension);
    // canvasContext.fillText(playerScore,100,100);
    // canvasContext.fillText(ComputerScore,canvas.width-100,100);      
};

，输出为：输出

Answer 1

尝试在没有括号的情况下从setInterval调用更新。