Question

关于一个HTML5CANVAS的问题！！！！！

<script>
var canvas;
var context;
window.onload = function() {
canvas = document.getElementById("myCanvas");
context = canvas.getContext("2d");
}

context.rect(20,20,150,100);
context.stroke();

</script>

为什么上面这串代码不正确

但是下面这样又可以运行了？？

<script>
var canvas;
var context;
canvas = document.getElementById("myCanvas");
context = canvas.getContext("2d");

context.rect(20,20,150,100);
context.stroke();

</script>

Answer 1

window.onload = function() {//页面加载完成才执行的代码
//
}
上面的不行，是因为这2个对象没有初始化，就调用方法，报错，可以这样写

window.onload = function() {
canvas = document.getElementById("myCanvas");
context = canvas.getContext("2d");
context.rect(20,20,150,100);
context.stroke();
}