`jQuery`是一个class
`$('p')`是一个jQuery实例
```
class jQuery {
constructor(selector) {
let slice = Array.prototype.slice
let dom = slice.call(document.querySelectorAll(selector))
let len = dom ? dom.length : 0
for (let i = 0; i < len; i++) {
this[i] = dom[i]
}
this.length = len
this.selector = selector || ''
}
append(node) {}
addClass(name) {}
html(data) {}
}
window.$ = function (selector) {
// 工厂模式
return new jQuery(selector)
}
```
