JavaScript（node.js）变量在内部函数调用时不可访问。now.js

我在一个节点项目中使用now.js和Mongoose，在访问Mongoose函数中的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"
    };
  });
};

---

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
    };
  });
};

---

在内部函数中，

this

可能被设置为其他值。因此，要保留要访问的值，可以将其分配给内部函数中可用的其他局部变量。

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
    };
  });
};

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`
};

在内部函数中，

this

可能被设置为其他值。因此，要保留要访问的值，可以将其指定给内部函数中可用的其他局部变量

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

将

this

的值设置为所需的值

---

使用

Function.prototype.bind

将

this

的值设置为您想要的值

ahhh brilliant这就是“that”的全部含义。ahhh brilliant这就是“that”的全部含义。这比jfriend00建议的使用“that”更好吗？@beck我称之为个人偏好。您还可以使用

.bind

来隧道额外的参数。（参见编辑）这种风格是否比jfriend00建议的使用“that”更好？@beck我称之为个人偏好。您还可以使用

.bind

来隧道额外的参数。（见编辑）