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1. 出现问题
(node:2171) UnhandledPromiseRejectionWarning: TypeError: Assignment to
constant variable. at getList
(/Users/dr/Desktop/Project/nodeBlog/src/controller/blog.js:10:9) at
handleBlogRouter (/Users/dr/Desktop/Project/nodeBlog/src/router/blog.js:18:30)
at getPostData.then.postData (/Users/dr/Desktop/Project/nodeBlog/app.js:56:28)
at process._tickCallback (internal/process/next_tick.js:68:7) (node:2171)
UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error
originated either by throwing inside of an async function without a catch
block, or by rejecting a promise which was not handled with .catch().
(rejection id: 1) (node:2171) [DEP0018] DeprecationWarning: Unhandled promise
rejections are deprecated. In the future, promise rejections that are not
handled will terminate the Node.js process with a non-zero exit code.
2. 问题分析
（1）源码
const blogResult = handleBlogRouter(req, res); if(blogResult){
blogResult.then(blogData => { res.end( JSON.stringify(blogData) ) }) return }
(2) 原因
const blogResult = handleBlogRouter(req, res)
定义了变量且存在初始值，而const的值会随着传入参数的不同发生变化，所以发生了报错：TypeError: Assignment to constant
variable.
3. 解决方法
将const 改为 let
4. const与let的比较
（1）区别：
* const一般是声明常量
如const a=1,const声明的变量不得改变值，这意味着，const一旦声明变量，就必须立即初始化，不能留到以后赋值。如const
a这样会报错
* let声明的变量可以改变，值和类型都可以改变，没有限制。
* const定义变量必须赋初始值，let不需要赋初始值
（2）共同点：
* let与const都是只在声明所在的块级作用域内有效。