// no inertia:

if (keyPressed[68]){               // key 'D'
    player.x = player.x + 10;      // player moves 10px to the right(+10) each frame.
}
if (keyPressed[65]){               // key 'A'
    player.x = player.x - 10;      // player moves 10px to the left(-10) each frame.
}

//-----------------------------------

// inertia:

if (keyPressed[68]){               // key 'D'
    player.vx = player.vx + 1;     // move right: player accelerates when key pressed, speed(vx) increases.
}
if (keyPressed[65]){               // key 'A'
    player.vx = player.vx - 1;     // move left: speed(vx) increases in the negative direction, e.g. -1,-2,-3.
}
player.x = player.x + player.vx;   // player moves left or right depending on the speed(vx). 

player.vx = player.vx * 0.95       // friction: speed(vx) decreases every frame.