State Machines

Your player can idle, run, jump, fall, dash, attack, get hurt and die. Each of these behaves differently, and only some changes make sense (you can’t jump while dead). Managing this with booleans quickly becomes a bug farm. A finite state machine (FSM) tames it.

The boolean soup problem

// 😱 Which combinations are valid? isJumping && isDashing && isDead?
if (isJumping && !isDashing && !isHurt) { ... }
else if (isDashing && !isDead) { ... }
else if (onGround && !isJumping && isMoving && !isAttacking) { ... }

With 5 booleans there are 32 combinations, most of them nonsense, and every new feature touches every if. An FSM replaces all those flags with a single variable: state.

A finite state machine

• The object is in exactly one state at a time.
• Each state has its own behaviour (update logic) and its own transitions (when to switch, and to what).
• Optional enter/exit hooks run once on change — the perfect place to start an animation, play a sound, or apply a jump impulse.

function update(dt) {
  switch (state) {
    case 'idle':
      if (input.jump) change('jump')
      else if (input.move) change('run')
      break
    case 'run':
      player.vx = input.dir * SPEED
      if (input.jump) change('jump')
      else if (!input.move) change('idle')
      break
    case 'jump':
      applyGravity(dt)
      if (player.vy > 0) change('fall')
      break
    // ...
  }
}
function change(next) {
  exit(state)          // e.g. stop the run dust particles
  state = next
  enter(state)         // e.g. vy = -JUMP_SPEED; play('jump'); sfx('jump')
}

Where FSMs show up in games

• Player controllers — like the demo.
• Enemy AI — patrol → chase → attack → flee (lesson 18).
• Animation — each state plays a clip (lesson 11).
• Game flow — title → playing → paused → game over (lesson 20).