本文介绍了Html 5 canvas getElementById() 返回 null/undefined的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着跟版网的小编来一起学习吧！
问题描述
here's the code:
HTML:
<body onload="initializeMap()">
<div id="map_canvas" style="width:100%; height:100%; z-index:1"></div>
<canvas id="control" style="width:100%; height:100%; z-index:2">Does Not Support Canvas Element</canvas>
</body>
Javascript:
<script type="text/javascript">
var canvas = document.getElementById('control');
var context = canvas.getContext('2d');
function draw(){
context.font = "bold 12px sans-serif";
context.fillText("x", 248, 43);
}
</script>
the draw function is called after initialization of the google map so the DOM should have already loaded by then, correct? What might have I done incorrectly?
解决方案
The DOM has already been loaded when the draw-function is called, that is correct.
But the var canvas = document.getElementById('control');-line is evaluated before that, because it is not in the draw-function. It is executed immediately in the <head> of the document BEFORE the elements have been rendered.
I would suggest you change your init function to something like that
var canvas,context;
function initializeMap() {
canvas = document.getElementById('control');
context = canvas.getContext('2d');
}
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