Cacher is the code snippet organizer for pro developers

function interpose(x,xs){
  var r = [];
  var len = xs.length;
  for(var i=0; i<len; i++){
    r.push(xs[i]);
    r.push(x);
  }
  r.pop();
  return r;
}

interpose('x',[1,2,3,4]); //=> [ 1, "x", 2, "x", 3, "x", 4 ]