How to implement blinking with requestAnimationFrame?

Question

JavaScript

As with the standard js animation system using requestAnimationFrame:

function loop() {
  let now = new Date().getTime(),
    dt = now - (time || now)

  time = now

  game.step(dt)
  game.render()
}

Implement flashing something at a slow speed while the main game process moves at a faster speed?
for example the player runs fast and the heart blinks slowly