JavaScript (node.js) 变量在内部函数调用时无法访问。now.js

Question

我在节点项目中使用 now.js 和 Mongoose，并且在访问 mongoose 函数内的 this.now 对象时遇到问题。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

将代码更改为：

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
    };
  });
};

在你的内部函数中， this 可能被设置为其他内容。因此，为了保留您想要访问的值，您可以将其分配给内部函数中可用的不同局部变量。

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`
};

利用 Function.prototype.bind 设置价值 this 对于您想要的价值