Basic collision detection for Javascript

Algorithms to detect collision in 2D games depend on the type of shapes that can collide (e.g. Rectangle to Rectangle, Rectangle to Circle, Circle to Circle). Generally you will have a simple generic shape that covers the entity known as a "hitbox" so even though collision may not be pixel perfect, it will look good enough and be performant across multiple entities. This article provides a review of the most common techniques used to provide collision detection in 2D games.

Axis-Aligned Bounding BoxSection

One of the simpler forms of collision detection is between two rectangles that are axis aligned — meaning no rotation. The algorithm works by ensuring there is no gap between any of the 4 sides of the rectangles. Any gap means a collision does not exist.

var rect1 = {x: 5, y: 5, width: 50, height: 50}
var rect2 = {x: 20, y: 10, width: 10, height: 10}

if (rect1.x < rect2.x + rect2.width &&
   rect1.x + rect1.width > rect2.x &&
   rect1.y < rect2.y + rect2.height &&
   rect1.y + rect1.height > rect2.y) {
    // collision detected!
}

// filling in the values =>

if (5 < 30 &&
    55 > 20 &&
    5 < 20 &&
    55 > 10) {
    // collision detected!
}

Source: https://developer.mozilla.org/en-US/docs/Games/Techniques/2D_collision_detection