Javascript - Homing bullet -

i've been working on little project, , while can bullet home in on mouse, has strange behavior.

if bullet going left/right/up/down mouse, change direction , forth , continue fly away mouse instead of turning @ it. also, makes loops unnecessary, figure that's because i'm not keeping direction positive or negative (i wrong though).

http://codepen.io/evanward/pen/voegdo?editors=001

this code have updating singular bullet.

var target = app.mouse.subtract(app.bullet.pos),       angle = target.angle();    if(angle < 0) {     angle += math.pi * 2;   }   var delta = angle - app.bullet.dir;   if(math.abs(delta) > 180) {     if(delta < 0) {       delta += 180;       delta *= -1;     }     else {       delta -= 180;       delta *= -1;     }   }   if(app.bullet.dir > math.pi * 2) {     app.bullet.dir -= math.pi * 2;   }    app.bullet.dir += delta/10;   app.bullet.pos.x += math.cos(app.bullet.dir) * app.bullet.vel;   app.bullet.pos.y += math.sin(app.bullet.dir) * app.bullet.vel;