Promise.prototype.finally implementation

promise.prototype.finally.js

/**
 * I think this is correct implementation of promise.finally,
 * Following what synchronized try/catch/finally does 
 */
if(!Promise.prototype.finally) {
  Promise.prototype.finally = function(f) {
    var ret, thr, hasthr = false;
    return this.then(function(a){ ret = a; return f(); })
      .then(function(){ return ret; })
      .catch(function(a) { hasthr = true; thr = a; return f(); })
      .then(function(a){ if(hasthr) throw thr; else return a; });
  }
}