深入理解JavaScript函数作为对象的高级特性

// 简单实验 函数作为对象的存在
let fn = function () {}
fn.prop = 'fnProp'
console.log(fn.prop) // fnProp

// 1：函数缓存示例
let store = {
nextId: 1, // id
cache: {}, // 缓存
add (fn) {
// 如果函数中没有id属性那么就缓存
if (!fn.id) {
console.log(`begin add func ${fn.name}`)
fn.id = store.nextId ++
// 设置完缓存之后返回true
return !!(store.cache[fn.id] = fn)
} else {
console.log(`${fn.name} is already in cache`)
}
}
}
function storeCache() {}
store.add(storeCache) // begin add func storeCache
store.add(storeCache) // storeCache is already in cache

// 2: 缓存记忆函数
function isPrime (value) {
if (!isPrime.anwers) isPrime.anwers = {}
// 先从缓存里面取
if (isPrime.anwers[value] != null ) {
return isPrime.anwers[value]
}
// 开始进行判断和计算
let prime = value != 1
for (let index = 2; index < value; index++) {
if (value % index == 0) {
prime = false
break;
} 
}
// 保存计算出来的值
return isPrime.anwers[value] = prime
}
console.log(isPrime(5))
console.log(`从函数记忆中直接读取${isPrime.anwers[5]}`)

// 3:缓存记忆DOM元素
function getElements (name) {
if (!getElements.cache) getElements.cache = {}
return getElements.cache[name] = getElements.cache[name] || document.getElementsByTagName(name);
}
console.log(getElements('div')) // HTMLCollection
console.log(getElements.cache['div']) // HTMLCollection

// 4：伪造数组方法
// <input type="button" id="add" >
// <input type="button" id="remove" >
let elems = {
length: 0, //为了保存个数
add (elem) {
Array.prototype.push.call(this, elem)
},
gather (id) {
this.add(document.getElementById(id))
}
}
elems.gather('add')
elems.gather('remove')
console.log(elems[0]); // <input type="button" id="add" >
console.log(elems[1]); // <input type="button" id="remove" >
console.log(elems.length); // 2
console.log(elems);
/**
0: input#add
1: input#remove
add: ƒ add(elem)
gather: ƒ gather(id)
length: 2
*/