How do I give this spaceship acceleration?

Question

Have a very small snippet of an asteroids-like game I\'m working on using only the DOM without Canvas. I have the \"ship\" moving pretty smoothly when arrow keys are pressed

Answer 1

 window.onkeydown = function( e ) {
    var kc = e.keyCode;
    e.preventDefault();

    if ( kc === 37 ) Keys.left++;
    else if ( kc === 38 ) Keys.up++;
    else if ( kc === 39 ) Keys.right++;
    else if ( kc === 40 ) Keys.down++;
  };
window.onkeyup = function(e)
{
  var kc = e.keyCode;
    e.preventDefault();

    if ( kc === 37 ) {Keys.left = 0;}
    else if ( kc === 38 ) Keys.up = 0;
    else if ( kc === 39 ) Keys.right = 0;
    else if ( kc === 40 ) Keys.down = 0;
}
function update() {
  if ( Keys.up ) {
    document.querySelector( 'div' ).style.transform += 'translateY( -'+Keys.up+'px )';
  }
  else if ( Keys.down ) {
    document.querySelector( 'div' ).style.transform += 'translateY( '+Keys.down+'px )';
  }

  if ( Keys.left ) {
    document.querySelector( 'div' ).style.transform += 'rotate( -'+Keys.left+'deg )';
  }
  else if ( Keys.right ) { 
    document.querySelector( 'div' ).style.transform += 'rotate( '+Keys.right+'deg )';
  }

    requestAnimationFrame( update );
}
requestAnimationFrame( update );