JavaScript（node.js）變量在內部函數調用時不可訪問。 now.js

Question

我在節點項目中使用now.js和Mongoose，並且無法訪問mongoose函數內的this.now對象。例如。

everyone.now.joinDoc = function (project_id){ 
    this.now.talk(); //this will work 
    Project.findOne({'_id':project_id}, function(err, project){ 
    if(project){ 
     this.now.talk(); // this will not work "TypeError: Cannot call method 'bark' of undefined" 
    }; 
    }); 
}; 

Answer 1

更改代碼這樣：

everyone.now.joinDoc = function (project_id){ 
    this.now.talk(); // this will work 
    var that = this; // save 'this' to something else so it will be available when 'this' has been changed 
    Project.findOne({'_id':project_id}, function(err, project){ 
    if(project){ 
     that.now.talk(); // use local variable 'that' which hasn't been changed 
    }; 
    }); 
}; 

裏面你的內心功能，this可能是被設置到別的東西。因此，爲了保留您想要訪問的值，可以將其分配給內部函數中可用的其他局部變量。

Answer 2

everyone.now.joinDoc = function (project_id){ 
    this.now.talk(); // this will work 
    Project.findOne({'_id':project_id}, (function(tunnel, err, project){ 
    if(project){ 
     this.now.talk(); 
    }; 
    }).bind(this, "tunnel")); // overwrite `this` in callback to refer to correct `this` 
}; 

使用Function.prototype.bind到this值的值設置爲你想要