Canvas Game: Collision Detection

Question

For my education I have to make a basic game in HTML5 canvas. The game is a shooter game. When you can move left -> right and space is shoot. When I shoot the bullets will move up. The enemy moves down. When the bullet hits the enemy the enemy has to dissapear and it will gain +1 score. But the enemy will dissapear after it comes up the screen.

Demo: http://jordikroon.nl/test.html

space = shoot + enemy shows up

This is my code:

            for (i=0;i<enemyX.length;i++) {

                if(enemyX[i] > canvas.height) {

                    enemyY.splice(i,1);
                    enemyX.splice(i,1);
                 } else {

                    enemyY[i] += 5;
                    moveEnemy(enemyX[i],enemyY[i]);

                 }
            }

            for (i=0;i<bulletX.length;i++) {

                if(bulletY[i] < 0) {
                    bulletY.splice(i,1);
                    bulletX.splice(i,1);
                 } else {

                    bulletY[i] -= 5;
                    moveBullet(bulletX[i],bulletY[i]);

                    for (ib=0;ib<enemyX.length;ib++) {

                      if(bulletX[i] + 50 < enemyX[ib] ||
                              enemyX[ib] + 50 < bulletX[i] ||
                               bulletY[i] + 50 < enemyY[ib] ||
                               enemyY[ib] + 50 < bulletY[i]) 
                        {   
                            ++score;
                            enemyY.splice(i,1);
                            enemyX.splice(i,1);
                        }
                    }   
                 }
            }

Objects:

        function moveBullet(posX,posY) {
            //console.log(posY);
            ctx.arc(posX, (posY-150), 10, 0 , 2 * Math.PI, false);

        }

        function moveEnemy(posX,posY) {

            ctx.rect(posX, posY, 50, 50);
            ctx.fillStyle = '#ffffff';
            ctx.fill(); 
        }

Answer 1

ORI think I see a problem. You have the following code on your collision detection

                  if(bulletX[i] + 50 < enemyX[ib] ||
                          enemyX[ib] + 50 < bulletX[i] ||
                           bulletY[i] + 50 < enemyY[ib] ||
                           enemyY[ib] + 50 < bulletY[i]) 

This is straight ORs(||), and the +50 is on the less than side. That means that it should actually trigger as true whenever the bullet is not in the hitbox. I suspect that you instead want to have the +50s on the greater than side, and have the ORs(||) be ANDs(&&) instead.

Answer 2

Further to Ben's correction, for future reference it's better to do a line-line collision detection, rather than your point-box collision detection.

The reason for this is, say your enemy is small and your bullet is moving fast. There is always a chance the bullet will appear BEFORE the enemy in one frame, and then behind the enemy in the next, therefore never registering as a hit.

Testing for an intersect with the bullet path and an imaginary line on the enemy will be far more accurate.