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Property:
measureText:

This example returns the width of the text you select.
Canvas:
your browser does not support the canvas tag
Code:
var c=document.getElementById('myCanvas');
var ctx=c.getContext('2d');
ctx.font='30px Arial';
var m=ctx.measureText('Hello World');
ctx.fillText(m.width,10,30);
Click the property values above to see the result
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