Quadratic Bézier Curve: Calculate Points

I'd like to calculate a point on a quadratic curve. To use it with the canvas element of HTML5.

When I use the quadraticCurveTo() function in JavaScript, I have a source point, a target point and a control point.

How can I calculate a point on the created quadratic curve at let's say t=0.5 with "only" knowing this three points?