Draw a half circle with mouse on html5 canvas

//ctx is a canvas context
ctx.beginPath();
var x = 50;               // The x-coordinate.
var y = 50;               // The y-coordinate.
var radius = 20;          // The arc radius; you need to calculate it, if you want to set it using mouse
var startAngle = 0;                     // The starting point on the circle.
//endAngle in radius = angle * (Math.PI / 180)
var endAngle = 180 * (Math.PI / 180); // The end point on the circle, half a circle = 180 degrees
var clockwise =  false // The direction of drawing = clock wise
ctx.arc(x, y, radius, startAngle, endAngle, clockwise); // Create the arc path.
ctx.stroke();