Fast rectangle to rectangle intersection

This is how that code can be translated to JavaScript. Note that there is a typo in your code, and in that of the article, as the comments have suggested. Specifically r2->right left should be r2->right < r1->left and r2->bottom top should be r2->bottom < r1->top for the function to work.

function intersectRect(r1, r2) {  return !(r2.left > r1.right ||            r2.right < r1.left ||            r2.top > r1.bottom ||           r2.bottom < r1.top);}

Test case:

var rectA = {  left:   10,  top:    10,  right:  30,  bottom: 30};var rectB = {  left:   20,  top:    20,  right:  50,  bottom: 50};var rectC = {  left:   70,  top:    70,  right:  90,  bottom: 90};intersectRect(rectA, rectB);  // returns trueintersectRect(rectA, rectC);  // returns false

function intersect(a, b) {  return (a.left <= b.right &&          b.left <= a.right &&          a.top <= b.bottom &&          b.top <= a.bottom)}

This assumes that the top is normally less than bottom (i.e. that y coordinates increase downwards).

This is how the .NET Framework implements Rectangle.Intersect

public bool IntersectsWith(Rectangle rect){  if (rect.X < this.X + this.Width && this.X < rect.X + rect.Width && rect.Y < this.Y + this.Height)    return this.Y < rect.Y + rect.Height;  else    return false;}

Or the static version:

public static Rectangle Intersect(Rectangle a, Rectangle b){  int x = Math.Max(a.X, b.X);  int num1 = Math.Min(a.X + a.Width, b.X + b.Width);  int y = Math.Max(a.Y, b.Y);  int num2 = Math.Min(a.Y + a.Height, b.Y + b.Height);  if (num1 >= x && num2 >= y)    return new Rectangle(x, y, num1 - x, num2 - y);  else    return Rectangle.Empty;}