NodeJs函数等待回调返回值

Question

我想在函数调用之外打印var a 的值。如果我运行这个程序，它会打印出undefined。

var http = require('http');

var foo = function(req, res){
    var a = xyz('5',function(aa){
        console.log(aa);
        return aa;
    });
    res.end(a);
    console.log(a);
}

function xyz(arg,callback){
    var aa = 55;
    return callback(aa);
}

http.createServer(foo).listen(8000);
console.log('start server');

Answer 1

让你的函数异步，然后等待 xyz()

var http = require('http');

var foo = async function(req, res){
  var a = await xyz('5'); 
  res.end(a);
  console.log(a);
}

function xyz(arg){
  var aa = 55;
  return aa;
}

http.createServer(foo).listen(8000);
console.log('start server');