javascript中Canvas为null错误

Canvas is null error in javascript

本文关键字：null 错误 Canvas javascript      更新时间：2023-09-26

我有这个代码：

<!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为空错误

以下代码行存在两个问题：

var canvas = document.getElementById("canvas"),
  1. 它在画布元素被解析并添加到DOM之前运行
  2. 它使用了错误的ID

更改为：

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

并将整个<script>块移动到主体的末端，刚好在</body>之前。

html中画布的id与中使用的id不匹配

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

正确的HTML应该是：

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

这是因为你在头上对接了脚本标记，但它在末尾的body标记中。

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