Question

我有这段代码：

<!DOCTYPE html>
<html>
 <head>
 <style type="text/css">
 canvas{border:#666 1px solid;}
</style>
<script type="text/javascript">

var canvas = document.getElementById("canvas"),
        context = canvas.getContext("2d"),
        playerimage = new Image(),
        x = canvas.width / 2, //align to centre of the screen
        y = canvas.height / 2, //same as above
        speed = 5, //speed for the player to move at
        width = 50, //width of the player
        height = 50; //height of the player

  function init() {


   playerimage.src = "ninja.png"; //path to the image to use for the player
   canvas.addEventListener("keypress", update);
  }

  function update(event) {
    if (event.keyCode == 38) {
        y -= speed; //going up
    }
    if (event.keyCode == 40) {
        y += speed; //going down
    }
    if (event.keyCode == 37) {
        x -= speed; //going left
    }
    if (event.keyCode == 39) {
        x += speed; //going right
    }
    render();
}

function render() {
   // context.clearRect(0, 0, canvas.width, canvas.height);
    context.drawImage(playerimage, x, y, width, height);

}
   </script>
 </head>
    <body onload="init();">
   <button onclick="init();">Draw</button>
    <canvas id="Mycanvas" width="550" height="400"></canvas>
   </body>  
  </html>

javascript控制台总是给我canvas是空错误

Answer 1

以下代码行有两个问题：

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

在解析canvas元素并将其添加到DOM之前运行。
使用错误的ID。

将其更改为：

var canvas = document.getElementById("Mycanvas"),

...并将整个<script>块移动到正文的末尾，就在</body>之前。

Answer 2

您在html中的画布ID与您在

中使用的ID不匹配

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

正确的HTML应该是：

<canvas id="canvas" width="550" height="400"></canvas>

在javascript中Canvas是null错误

2 个答案: