如何聆聽Javascript中的變量更改？

Question

我一直在使用Node.js和CouchDB。我想要做的是在一個對象內進行數據庫調用。下面是我在看現在的情形：

var foo = new function(){ 
    this.bar = null; 

    var bar; 

    calltoDb(... , function(){ 

     // what i want to do: 
     // this.bar = dbResponse.bar; 

     bar = dbResponse.bar;  

    }); 

    this.bar = bar; 

} 

與所有這一切的問題是，CouchDB的回調是異步的，「this.bar」現在是回調函數的範圍之內，不班上。有沒有人有任何想法來完成我想要的？我不希望有一個處理程序對象必須使數據庫調用對象，但現在我真的難以解決它是異步的問題。

Answer 1

參考只要保持在this各地：

function Foo() { 
    var that = this; // get a reference to the current 'this' 
    this.bar = null; 

    calltoDb(... , function(){ 
     that.bar = dbResponse.bar; 
     // closure ftw, 'that' still points to the old 'this' 
     // even though the function gets called in a different context than 'Foo' 
     // 'that' is still in the scope and can therefore be used 
    }); 
}; 

// this is the correct way to use the new keyword 
var myFoo = new Foo(); // create a new instance of 'Foo' and bind it to 'myFoo' 

Answer 2

保存一個參考this，像這樣：

var foo = this; 
calltoDb(... , function(){ 

    // what i want to do: 
    // this.bar = dbResponse.bar; 

    foo.bar = dbResponse.bar;  

});