Functions as Objects.md

Functions as Objects

1. 函数和对象有多相似?

   var obj = {}
   var fn = function () {}
   console.log(obj && fn, '对象和函数都存在')
   // function () {} '对象和函数都存在'

   var obj = {}
   var fn = function () {}
   obj.prop = 'some value'
   fn.prop = 'some value'
   console.log(obj.prop === fn.prop, '都是对象,都有属性')
   // true 都是对象,都有属性

2. 函数可以缓存返回值吗?

   // 原函数
   function getElements (name) {
     var results
     if (getElements.cache[name]) {
       results = getElements.cache[name]
     } else {
       results = documents.getElementsByTagName(name)
       getElements.cache[name] = results
     }
     return results
   }
   getElements.cache = {}
   console.log('Elements found: ', getElements('pre').length )
   console.log('Cache found: ', getElements.cache.pre.length)
   
   // 我们改造下，方便演示
   function square(a) {
     let result
     if (square.cache[a]) {
       result = square.cache[a]
     } else {
       result = a * a
       square.cache[a] = result
     }
     return result
   }
   square.cache = {}
   square(2)
   console.log(square.cache)
   // 4
   // {2: 4}

3. 一种合理的方式去缓存结果

   function isPrime (num) {
     if (isPrime.cache[num] !== null) {
       return isPrime.cache[num]
     }
     var prime = num !== 1
     for (var i = 2; i < num; i++) {
       if (num % i === 0) {
         prime = false
         break
       }
     }
     isPrime.cache[num] = prime
     return prime
   }
   isPrime.cache = {}
   console.log(isPrime(5), '确认函数工作了, 5是最好的')
   console.log(isPrime.cache[5], '确认结果已缓存')
   // true 确认函数工作了, 5是最好的
   // {5: true} 确认结果已缓存