我的红色方块不会出现，因为："speedX and speedY aren't defined"-console

Question

所以我正在学习这个 javascript 的游戏制作教程，但我真的找不到我做了什么才能让我的红色方块不出现在屏幕上。事实上，我确实认为这是速度问题，因为当我在控制台中更改对象的速度时，它出现...然后飞出屏幕。

这是我的代码：

    <!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
<style>
canvas {
    border:1px solid #d3d3d3;
    background-color: #f1f1f1;
}
</style>
</head>
<body onload="startGame()">
<script>

var myGamePiece;

function startGame(){
    myGamePiece = new component(30, 30, "red", 10, 120);
    myGameArea.start();
}

var myGameArea = {
    canvas: document.createElement("canvas"),
    start: function(){
        this.canvas.width = 480;
        this.canvas.height = 270;
        this.context = this.canvas.getContext("2d");
        document.body.insertBefore(this.canvas, document.body.childNodes[0]);
        this.interval = setInterval(updateGameArea, 20);
    },
    clear: function(){
        this.context.clearRect(0, 0, this.canvas.width, this.canvas.height);
    }
}

function component(width, height, color, x, y){
    this.width = width;
    this.height = height;
    this.speedX = 0;
    this.speedY = 0;
    this.x = x;
    this.y = y;
    this.update = function(){
        ctx = myGameArea.context;
        ctx.fillStyle = color;
        ctx.fillRect(this.x, this.y, this.width, this.height);
    }
    this.newPos = function() {
        this.x += speedX;
        this.y += speedY;
    }
}

function updateGameArea(){
    myGameArea.clear();
    myGamePiece.newPos();
    myGamePiece.update();
}

function moveup(){
    myGamePiece.speedY -= 1;
}
function movedown(){
    myGamePiece.speedY += 1;
}
function moveright(){
    myGamePiece.speedX += 1;
}
function moveleft(){
    myGamePiece.speedX -= 1;
}



    </script>

    <button onmousedown="moveup()">UP</button>
    <button onmousedown="movedown()">DOWN</button>
    <button onmousedown="moveright()">RIGHT</button>
    <button onmousedown="moveleft()">LEFT</button>

</body>
</html>

感谢您的帮助！

Answer 1

 this.newPos = function() {
    this.x += speedX;
    this.y += speedY;

控制台正确，speedX和speedY不存在

一共有三种解决方案：

解决方案一：使用that

var 那 = 这个； // 保存 this

的本地副本

 this.newPos = function() {
    that.x += that.speedX;
    that.y += that.speedY;
 }

方案二：使用绑定：

this.newPos = function() {
     this.x += this.speedX;
     this.y += thisspeedY;
}.bind(this);

解决方案 3：ES6 - 使用箭头函数

this.newPos = () => {
     this.x += this.speedX;
     this.y += thisspeedY;
 };

我的红色方块不会出现，因为："speedX and speedY aren't defined"-console

My red square wont show up because: "speedX and speedY aren't defined"-console

javascript

debugging

2d-games