raf.js

var rAF = "equestAnimationFrame";

window["r"+rAF] = window["r"+rAF] ||
    window["webkitR"+rAF] ||
    window["mozR"+rAF] ||
    window["msR"+rAF] ||
    window["oR"+rAF];


// A list
[ "r", "webkitR", "mozR", "msR", "oR", "" ].join( rAF + "," ).split( "," )


// Best one yet:
[ "r", "webkitR", "mozR", "msR", "oR" ].reduce(function( p, v ) {
  return window[ v + p ] || p;
}, "equestAnimationFrame");




["r","webkitR","mozR","msR","oR"].reduce(function(p, v) { return window[v + p] || p; }, "equestAnimationFrame");




// Alternate approach ideas

var suffix = "equestAnimationFrame",
  rAF = [ "r", "webkitR", "mozR", "msR", "oR" ].filter(function( val ) {
    return val + suffix in window;
  })[ 0 ] + suffix;
/*
var suffix="equestAnimationFrame",rAF=["r","webkitR","mozR","msR","oR"].filter(function(a){return a+suffix in window})[0]+suffix;

Original Size:  179 bytes (164 bytes gzipped)
Compiled Size:  129 bytes (131 bytes gzipped)
Saved 27.93% off the original size (20.12% off the gzipped size)
*/


var prefix = function( window ) {
  var raf = "equestAnimationFrame",
    idx;

  for ( var prop in window ) {
    idx = prop.indexOf( raf );

    if ( idx > -1 ) {
      return prop.slice( 0, idx - 1 );
    }
  }

  return null;
}( this );

/*
var prefix=function(c){var a,b;for(b in c)if(a=b.indexOf("equestAnimationFrame"),a>-1)return b.slice(0,a-1);return null}(this);

Original Size:  232 bytes (184 bytes gzipped)
Compiled Size:  127 bytes (130 bytes gzipped)
Saved 45.26% off the original size (29.35% off the gzipped size)
*/

console.log(
  prefix
);