Canvas disappearing

I’ve written a simple script in JS. When I put hit it into html document (at the bottom of body tag), it works fine. When I try to put it into external document and I link it, script runs but the canvas element disappears (just checked: it works when I put the script in the bottom, but not in head tag).
This version works:

			var c = document.getElementById("canvasId");
			var ctx = c.getContext("2d"); 
			var text = "Nazwa pomiaru";
			var boxWidth = 200;
			var boxHeight = 200;
			var minValue = 0;
			var maxValue = 100;
			var midValue = (Math.abs(minValue) + Math.abs(maxValue))/2;
			var unit = "%";
			var indicWidth = 50;
			var indicHeight = 0.6 * boxHeight;
			var textPosition = 0.875;
			var leftMargin = 0.375;
			var topMargin = 0.15;
			var bottomMargin = 0.75;
			var value = Math.floor(Math.random() * 101);
			var warningTop = 90;
			var dangerTop = 95;
			var warningBottom = 10;
			var dangerBottom = 5;
			var thresholdTrasparency = 0.6;
			var strokeColor = "#CCCCCC";
			var valueColor = "#993333";
			var warningColor = "#ff6600";
			var dangerColor = "#cc0000";
			var textColor = "#000000";
			var textColor2 = "#ffffff";
			ctx.beginPath();
			ctx.fillText((maxValue + unit), (leftMargin * boxWidth - ctx.measureText(maxValue + unit).width-1), (topMargin * boxHeight + 3), (boxWidth / 4)); //draw a maxValue
			ctx.fillText((midValue + unit), (leftMargin * boxWidth - ctx.measureText(midValue + unit).width-1), ((topMargin + bottomMargin) / 2 * boxHeight + 3), (boxWidth / 4)); //draw a midValue
			ctx.fillText((minValue + unit), (leftMargin * boxWidth - ctx.measureText(minValue + unit).width-1), (bottomMargin * boxHeight + 3), (boxWidth / 4));	//draw a minValue		
			ctx.fillText(text, ((boxWidth - ctx.measureText(text).width) / 2), (textPosition * boxHeight), (textPosition * boxWidth)); //type a text
			ctx.lineWidth = "0";
			ctx.globalAlpha = thresholdTrasparency;
			if (warningTop !== 0 || warningBottom !== 0) {
				ctx.fillStyle = warningColor;
				ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight), indicWidth, ((100 - warningTop) * (indicHeight / 100))); //draw a warningTop
				ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight + (100 - warningBottom) * (indicHeight / 100)), indicWidth, (indicHeight - (100 - warningBottom) * (indicHeight / 100))); //draw a warningBottom 
			}
			if (dangerTop !== 0 || dangerBottom !== 0) {
				ctx.fillStyle = dangerColor;
				ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight), indicWidth, ((100 - dangerTop) * (indicHeight / 100))); //draw a dangerTop
				ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight + (100 - dangerBottom) * (indicHeight / 100)), indicWidth, (indicHeight - (100 - dangerBottom) * (indicHeight / 100))); //draw a dangerBottom 
			}
			ctx.beginPath();
			ctx.globalAlpha = 1;
			ctx.fillStyle = valueColor;
			ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight + (100 - value) * (indicHeight / 100)), indicWidth, (indicHeight - (100 - value) * (indicHeight / 100))); //draw a valueIndicator
			if (value <= 47) {
				ctx.fillStyle= textColor;
			}
			else {
				ctx.fillStyle = textColor2;
			}
			ctx.fillText(value, ((boxWidth - ctx.measureText(value).width) / 2), (boxHeight / 2));
			ctx.shadowBlur = 6;
			ctx.shadowColor = "#000000";
			ctx.lineWidth="2";
			ctx.strokeStyle = strokeColor;
			ctx.strokeRect((leftMargin * boxWidth), (topMargin * boxHeight), indicWidth, indicHeight); //draw a rectangle
			ctx.shadowBlur = 0;
			document.write(value); 
<!doctype html>
<html>
	<head>
		<title>Slider</title>
	</head>
	<body>
		<canvas id="canvasId" style="border: 3px solid; background: #ffffff;" width="200px" height="200px">
		No support.
		</canvas>
		<script src="canvas.js"></script>
	</body>
</html>

This version does not work:

function draw () {
			var c = document.getElementById("canvasId");
			var ctx = c.getContext("2d"); 
			var text = "Nazwa pomiaru";
			var boxWidth = 200;
			var boxHeight = 200;
			var minValue = 0;
			var maxValue = 100;
			var midValue = (Math.abs(minValue) + Math.abs(maxValue))/2;
			var unit = "%";
			var indicWidth = 50;
			var indicHeight = 0.6 * boxHeight;
			var textPosition = 0.875;
			var leftMargin = 0.375;
			var topMargin = 0.15;
			var bottomMargin = 0.75;
			var value = Math.floor(Math.random() * 101);
			var warningTop = 90;
			var dangerTop = 95;
			var warningBottom = 10;
			var dangerBottom = 5;
			var thresholdTrasparency = 0.6;
			var strokeColor = "#CCCCCC";
			var valueColor = "#993333";
			var warningColor = "#ff6600";
			var dangerColor = "#cc0000";
			var textColor = "#000000";
			var textColor2 = "#ffffff";
			ctx.beginPath();
			ctx.fillText((maxValue + unit), (leftMargin * boxWidth - ctx.measureText(maxValue + unit).width-1), (topMargin * boxHeight + 3), (boxWidth / 4)); //draw a maxValue
			ctx.fillText((midValue + unit), (leftMargin * boxWidth - ctx.measureText(midValue + unit).width-1), ((topMargin + bottomMargin) / 2 * boxHeight + 3), (boxWidth / 4)); //draw a midValue
			ctx.fillText((minValue + unit), (leftMargin * boxWidth - ctx.measureText(minValue + unit).width-1), (bottomMargin * boxHeight + 3), (boxWidth / 4));	//draw a minValue		
			ctx.fillText(text, ((boxWidth - ctx.measureText(text).width) / 2), (textPosition * boxHeight), (textPosition * boxWidth)); //type a text
			ctx.lineWidth = "0";
			ctx.globalAlpha = thresholdTrasparency;
			if (warningTop !== 0 || warningBottom !== 0) {
				ctx.fillStyle = warningColor;
				ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight), indicWidth, ((100 - warningTop) * (indicHeight / 100))); //draw a warningTop
				ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight + (100 - warningBottom) * (indicHeight / 100)), indicWidth, (indicHeight - (100 - warningBottom) * (indicHeight / 100))); //draw a warningBottom 
			}
			if (dangerTop !== 0 || dangerBottom !== 0) {
				ctx.fillStyle = dangerColor;
				ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight), indicWidth, ((100 - dangerTop) * (indicHeight / 100))); //draw a dangerTop
				ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight + (100 - dangerBottom) * (indicHeight / 100)), indicWidth, (indicHeight - (100 - dangerBottom) * (indicHeight / 100))); //draw a dangerBottom 
			}
			ctx.beginPath();
			ctx.globalAlpha = 1;
			ctx.fillStyle = valueColor;
			ctx.fillRect((leftMargin * boxWidth), (topMargin * boxHeight + (100 - value) * (indicHeight / 100)), indicWidth, (indicHeight - (100 - value) * (indicHeight / 100))); //draw a valueIndicator
			if (value <= 47) {
				ctx.fillStyle= textColor;
			}
			else {
				ctx.fillStyle = textColor2;
			}
			ctx.fillText(value, ((boxWidth - ctx.measureText(value).width) / 2), (boxHeight / 2));
			ctx.shadowBlur = 6;
			ctx.shadowColor = "#000000";
			ctx.lineWidth="2";
			ctx.strokeStyle = strokeColor;
			ctx.strokeRect((leftMargin * boxWidth), (topMargin * boxHeight), indicWidth, indicHeight); //draw a rectangle
			ctx.shadowBlur = 0;
			document.write(value); 
};

window.onload = draw;
<!doctype html>
<html>
	<head>
		<title>Slider</title>
	</head>
	<body>
		<canvas id="canvasId" style="border: 3px solid; background: #ffffff;" width="200px" height="200px">
		No support.
		</canvas>
		<script src="canvas.js"></script>
	</body>
</html>

The second version someway hides canvas tag but the text appears (so the script works).

Is there any solution for this?


Source: html5

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