如何在mongoose中设置检索回调，在全局变量中

问题描述

我想在全局变量中设置 db ，但是当我得到名称 c> findOne 函数显示我未定义，我该怎么办？

I want to set db in global variable, but when I get console name out of findOne function show me undefined, What can I do?

var name;

schema.findone({name : 'Bob'} , function(er , db){
  name = db;
  console.log(db);
});

console.log(name);

谢谢。

推荐答案

超级经典初学者对异步的错误：）

Super classic beginner mistake about asynchronism :)

发生了什么：

var name; // FIRST you declare the name variable

schema.findone({name : 'Bob'} , function(er , db){ // SECOND you launch a request to the DB
  name = db; // FOURTH name is populated.
  console.log(db);
});

console.log(name); // !! THIRD !! you log name - it's empty

你应该做什么：

schema.findone({name : 'Bob'} , function(er , db){
  doSomethingElse(db);
});

function doSomethingElse(name){
    console.log(name); // It's defined.
}

你甚至不会声明一个全局变量，因为这是一个糟糕的做法。
一旦数据可用，将其传递给另一个函数并执行一些操作。所以你不会污染你的全球范围。

You souldn't even declare a global variable, as it's a bad practice.As soon as the data is available, pass it to another function and do something with it. So you don't pollute your global scope.

编辑：由于某种原因你绝对需要一个全局变量，所以这样做： / p>

Edit : Since you absolutely want a global variable for some reason, then do this :

var name;

schema.findone({name : 'Bob'} , function(er , db){
  name = db;
  console.log(name); // works fine
  doSomethingElse();
});

console.log(name); // name is empty here, because the DB request is still in progress at this stage

function doSomethingElse(){
    console.log(name); // Tadaaaa! It's a global variable and is defined!
}

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