Calculates the shortest difference between two given angles given in radians, in JS

getDeltaAngle.js

// http://docs.unity3d.com/ja/current/ScriptReference/Mathf.DeltaAngle.html

var getDeltaAngle = function () {

  var TAU = 2 * Math.PI;
  var mod = function ( a, n ) { return ( a % n + n ) % n; }

  return function ( current, target ) {

  var a = mod( ( current - target ), TAU );
  var b = mod( ( target - current ), TAU );

    return a < b ? -a : b;

  }

}();

getDeltaAngle( 0, Math.PI );

Slightly clearer version (in my opinion):

var getDeltaAngle = function () {
  var TAU = 2 * Math.PI;
  var mod = function (a, n) { return ( a % n + n ) % n; } // modulo
  var equivalent = function (a) { return mod(a + Math.PI, TAU) - Math.PI } // [-π, +π]
  return function (current, target) {
    return equivalent(target - current);
  }
}();