1.手写一个memoize函数

纯函数的好处

function getPi(r){
  console.log('执行了')
  return Math.PI * r * r;
}

function memorize(fn){
  let cache = {}
  return function(){
    let str = JSON.stringify(arguments)
    cache[str] = cache[str] || fn.apply(null, arguments)
    return cache[str]
  }
}
let getCount = memorize(getPi)
console.log(getCount(4))
console.log(getCount(4)) 
console.log(getCount(4))