Node.js、Mongodb：如何使用 `promise.then` 设置全局变量？

Question

在代码中有一个promise.then试图被用来设置一个全局变量originalUrl，但是originalUrl没有被改变。任何帮助将不胜感激。我的应用程序.js：// create a mongoose modelvar urlSchema = new mongoose.Schema({  original_url: String,  short_url: String});urlSchema.plugin(findOrCreate)var Url = mongoose.model('Url', urlSchema);let promise = new Promise(function(resolve, reject) {  app.post("/api/shorturl/new", (req, res) => {    // receive an url in the post and return a    // short id. After that, the short id can be used    // to go to the original url.    // Handle the data in the POST request    // get the hostname from the request        originalUrl = req.body.url;    // Create an identifier for the url    shortUrl = md5(originalUrl).toString().slice(-7)          // test to see if the host is live    var url = new URL(originalUrl);    hostname = url.hostname;    // if host is live    dns.lookup(hostname, function (err) {      if (err == undefined){        // if not found, save the original_url and the         // short_url, return          Url.findOrCreate({ original_url: originalUrl, short_url: shortUrl}, (err, result) => {          console.log(result)          res.json({          original_url: originalUrl,          short_url: shortUrl          });         })      }      if (err != undefined){        console.log(">>> error <<<")        res.json({"error":"host down"});      }    });    resolve(originalUrl)  });})promise.then(  function(result) { originalUrl = result },  function(error) { /* handle an error */ });console.log(originalUrl) // undefined

Answer 1

传递给的函数中的代码then()在承诺解决之前不会运行。所以不是这个：

promise.then(

  function(result) { originalUrl = result },

  function(error) { /* handle an error */ }

);

console.log(originalUrl)

...你需要做更多这样的事情：

promise.then(

  function(result) { originalUrl = result },

  function(error) { /* handle an error */ }

).then(

  function() { console.log(originalUrl); }

);

这是一个简单的可运行示例，希望可以让您了解其工作原理：

var originalUrl = undefined;

var promise = new Promise((resolve, reject) => {

  setTimeout( function() {

    resolve("http://example.com");

  }, 250) 

});

promise.then(

  function(result) { originalUrl = result },

  function(error) { /* handle an error */ }

).then(

  function() { console.log('promise resolved: ', originalUrl); }

);

console.log('promise pending: ', originalUrl);

结果：

promise pending:  undefined

promise resolved:  http://example.com