canvas.height 不被接受为初始变量值

var pos_y = canvas.height;- 这样的条目无效（整个画布立即变为空白）。当您尝试设置高度时document.getElementById。

在这种情况下，方法canvas接受canvas.height. 为什么代码的有效性甚至会删除一些无害的东西 var h = document.getElementById('drawingCanvas').clientHeight;

var canvas;
    var context;
        window.onload = function() {
            canvas = document.getElementById("drawingCanvas");
            context = canvas.getContext("2d");        
            setTimeout("drawFrame()", 300);         
        }   
            var pos_y = 200; // не принимает canvas.height

        function drawFrame(){           
            context.clearRect(0, 0, canvas.width, canvas.height);           
            context.fillStyle = 'yellow';
            context.strokeStyle = 'yellow'
            context.fill();                         
            context.beginPath();
            context.arc(canvas.width/2, pos_y, 30, 0,Math.PI*2, true);
            context.stroke();
             pos_y-=1;      
            setTimeout("drawFrame()", 300);    
        }