!!的作用

!!是一个很简单的方式，让任意JavaScript表达式变成Boolean的方式， 例如：!!“hello” === true and !!0 === false
参考: 忍者禁术

var store = { 
nextId: 1, cache: {}, 
add: function(fn){ if (!fn.id){ fn.id = store.nextId++; 
return !!(store.cache[fn.id] = fn); } } } 
  function ninja(){}; 
  assert(store.add(ninja), "Function was safely added."); 
  assert(!store.add(ninja), "But it was only added once.");