To find the direction from one object to another, we need to use some trig functions. First we need to know the positions of those objects. Then we can find the change in X and Y. From there we can find the distance between them and the angle from one to the other.

Here’s an example: Point A is located at coordinates (-10, -20), and Point B is located at (110, 70). To go from A to B is a change of (120, 90).

To find the angle from A to B, we use the arc-tangent function.

var distX = objectB.x - objectA.x;
var distY = objectB.y - objectA.y;
 
var angle_radians = Math.atan2(distY, distX);
var angle_degrees = radiansToDegrees( angle_radians );

function radiansToDegrees(r){
    return r * 180 / Math.PI;
}

In Javascript and ActionScript, the Math function atan2 will take y and x, and return the direction in radians. That means the returned value will be between -π and π. to convert radians to degrees, we multiply by 180 and divide by π. In this example, the angle is about 36.87°.

To find the distance between them, we use the Pythagorean theorem: a² + b² = c², or:

var dist = Math.sqrt(distX * distX + distY * distY);

In this example, the distance is 150.

Now you can set the rotation of the first object to look at the second object. OK, but how do you move it forward at a constant rate? If you update its x and y position by 120 and 90, respectively, it would reach the goal in one step. Not much of an animation. Instead, give it a speed and use trigonometry to figure out the change in the x and y positions.

var speed = 6;
 
var deltaX = Math.cos(angle_radians) * speed;
var deltaY = Math.sin(angle_radians) * speed;
 
objectA.x += deltaX;
objectA.y += deltaY;

First we decide what speed to use. This is in pixels per frame. Each frame the object will move by 6 pixels in its forward direction. Then each frame, calculate the change in the x and y positions using the cosine and sine functions. This time we need to use radians, not degrees. These functions will return a value between -1 and 1. Now just multiply by the speed and apply it to the object.

To make the object turn smoothly, as in the example above, is a little more complicated, so I’ll save that for another time. But to give you a hint, it starts with a turning speed. You can see how it’s done in the attached FLA: trigTest.fla