I'm just playing with mithril until I made a pen tool :D

canvas.ts

export const Canvas: Mithril.FactoryComponent = () => {

    let canvas: HTMLCanvasElement, context: CanvasRenderingContext2D;

    let points: number[] = [];
    let data: ImageData;

    function onMouseDown(e: MouseEvent) {
        data = context.getImageData(0, 0, canvas.width, canvas.height);
        points.push(e.offsetX, e.offsetY);
    }
    
    function onMouseUp(e: MouseEvent) {
        points.splice(0);
    }

    function onMouseMove(e: MouseEvent) {
        if (points.length === 0) return;
        points.push(e.offsetX, e.offsetY);
        context.putImageData(data, 0, 0);
        context.beginPath();
        for (let i = 0; i < points.length; i += 2) {
            const x = points[i],
                  y = points[i + 1];
            if (i === 0) context.moveTo(x, y);
            else         context.lineTo(x, y);
        }
        context.stroke();
    }
  
    return {
  
      oncreate({ dom }) {
        canvas = dom as HTMLCanvasElement;
        context = canvas.getContext('2d');
      },

      view() {
        return m('canvas', {
            onmousedown: onMouseDown,
            onmouseup: onMouseUp,
            onmousemove: onMouseMove,
            onmouseout: onMouseUp,
        });
      }
  
    };
  
  };