title: JavaScript simple unit conversion with unit selection
body: <p>I'm trying to build a unit converter with a dropdown selection of units but I'm struggling to find a simple example. Could anyone share their knowledge, an example or direct me to a tutorial?
  I'm new to JavaScript and have looked at the source code of several conversion websites and understand I need to use an array but I can't seem to following how the code looks at the user input unit then converts it using the factors in the array to the selected output unit. I would like a step by step guide of some type.
  Any help would be much appreciated.</p>