title: Making mousemove fire only when user is holding down the left mouse button
body: <p>I'm making a 360 degree image viewer (in plain JavaScript) and i'm trying to make it so that when a user mouses left or right over the image it swaps images (creating the 360 effect) but only while they're holding down their mouse button.</p>
  
  <p>I tried using the event's "which" property (equals 1 in Chrome when left mouse button is held but seems to always equal 1 in Safari and FF even when the mouse button isn't being held).</p>