请教jQuery源码中Sizzle模块的createCache函数

不明白为什么这个函数能实现缓存？
第一：keys是局部变量，每次调用createCache()，keys都被初始化为空数组。
第二：cache闭包函数每次通过createCache()(key, value)即便接收了属性名和属性值key和value;
但返回的只是属性值, 每次createCache()也得不到属性值呀, 因为每次调用cache都被重新声明, 也不可能保存前一个alue值呀.

/**

• Create key-value caches of limited size
• @returns {Function(string, Object)} Returns the Object data after storing it on itself with
• property name the (space-suffixed) string and (if the cache is larger than Expr.cacheLength)
• deleting the oldest entry
  */
  function createCache() {
  var keys = [];
  function cache( key, value ) {
  // Use (key + " ") to avoid collision with native prototype properties (see Issue #157)
  if ( keys.push( key + " " ) > Expr.cacheLength ) {
  // Only keep the most recent entries
  delete cache[ keys.shift() ];
  }
  return (cache[ key + " " ] = value);
  }
  return cache;
  }