In Gaffer's "Fix Your Timestep!" article, the author explains how to free your physics' loop from the paint one.

Here is the final code, written in C:

double t = 0.0;
const double dt = 0.01;

double currentTime = hires_time_in_seconds();
double accumulator = 0.0;

State previous;
State current;

while ( !quit )
{
     double newTime = time();
     double frameTime = newTime - currentTime;
     if ( frameTime > 0.25 )
          frameTime = 0.25;   // note: max frame time to avoid spiral of death
     currentTime = newTime;

     accumulator += frameTime;

     while ( accumulator >= dt )
     {
          previousState = currentState;
          integrate( currentState, t, dt );
          t += dt;
          accumulator -= dt;
     }

     const double alpha = accumulator / dt;

     State state = currentState*alpha + previousState * ( 1.0 - alpha );

     render( state );
}

I'm trying to implement this in JavaScript but I'm quite confused about the second while loop...

Here is what I have for now (simplified):

...
(function animLoop(){
  ...

  while (accumulator >= dt) { // While? In a requestAnimation loop? Maybe if?
    ...
  }

  ...
  // render

  requestAnimationFrame(animLoop); // stand for the 1st while loop [OK]
}())

As you can see, I'm not sure about the while loop inside the requestAnimation one...

I thought replacing it with a if but I'm not sure it will be equivalent...

Maybe some can help me.