性能優(yōu)化function updateBalls(){

??? for( var i = 0 ; i < balls.length ; i ++ ){

??????? balls[i].x += balls[i].vx;
??????? balls[i].y += balls[i].vy;
??????? balls[i].vy += balls[i].g;

??????? if( balls[i].y >= WINDOW_HEIGHT-RADIUS ){
??????????? balls[i].y = WINDOW_HEIGHT-RADIUS;
??????????? balls[i].vy = - balls[i].vy*0.75;
??????? }
??? }

??? var cnt = 0
??? for( var i = 0 ; i < balls.length ; i ++ )
??????? if( balls[i].x + RADIUS > 0 && balls[i].x -RADIUS < WINDOW_WIDTH )
??????????? balls[cnt++] = balls[i]

??? while( balls.length > Math.min(300,cnt) ){
??????? balls.pop();
??? }
}