提一个优化 if 的方法
可以这样写：
const condition = condition1
let obj ={
'condition1':()=>{...},
'condition2':()=>{...},
'condition3':()=>{...},
}
obj[condition]()
实例：
const condition =2
let obj ={
'1':()=>{ document.write(1)},
'2':()=>{ document.write(2)},
'3':()=>{ document.write(3)},
}
obj[condition]()
