simple_cache.js

var cache = {};
function checkCache(key){
  if(cache[key]){
    return cache[key];
  } else {
    // do some long/expensive action
    // ex: calculation, ajax, etc...
    cache[key] = Math.random();
    return cache[key];
  }
}

timed_cache.js

// This will keep it in the cache unless
// it is older than 5 seconds

var cache = {};
function checkCache(key){
  var now = +new Date();

  var found = cache[key];
  if(found && isNotOld(now, found.ts)){
    return cache[key];
  } else {
    cache[key] = {
      val: Math.random(),
      ts: +new Date()
    };

    return cache[key];
  }
}

function isNotOld(t1, t2){
  var lifetime = 5000;
  return Math.abs(t1-t2) < 5000
}