Servo does not fully rotate to 0 or 180 degrees

Right now, I'm powering the Espruino via the USB and have the servo sharing the ground and powered via a separate 5v power supply like the docs recommend.

Below is the code I'm using. It's pretty basic. I was just trying to test out the servo module.

var hat = require('servo').connect(A0);
var pos=0,t=null;

function sweep(){
  hat.move(pos,500,function(){
    if(pos===0){pos=1;}else{pos=0;}
    t=setTimeout(function(){ sweep(); },1000);
    
  });
}
hat.move(0.5);
sweep();