function polarToCartesian(centerX, centerY, radius, angleInDegrees) {

  var angleInRadians = (angleInDegrees-90) * Math.PI / 180.0;

​

  return {

    x: centerX + (radius * Math.cos(angleInRadians)),

    y: centerY + (radius * Math.sin(angleInRadians))

  };

}

​

function describeArc(x, y, radius, startAngle, endAngle){

​

    var start = polarToCartesian(x, y, radius, endAngle);

    var end = polarToCartesian(x, y, radius, startAngle);

​

    var largeArcFlag = endAngle - startAngle <= 180 ? "0" : "1";

​

    var d = [

        "M", start.x, start.y, 

        "A", radius, radius, 0, largeArcFlag, 0, end.x, end.y

    ].join(" ");

​

    return d;       

}

​

window.onload = function() {

  document.getElementById("arc1").setAttribute("d", describeArc(150, 100, 50, -90, +90));

};

​