回调函数中的`this`

Question

我以前用过

MyClass.prototype.myMethod1 = function(value) {
    this._current = this.getValue("key", function(value2){
        return value2;
    });
};

如何在回调函数中访问此值，如下所示？

MyClass.prototype.myMethod1 = function(value) {
   this.getValue("key", function(value2){
       //ooopss! this is not the same here!    
       this._current = value2;
   });
};

Answer 1

MyClass.prototype.myMethod1 = function(value) {
    var that = this;
    this.getValue("key", function(value2){
         //ooopss! this is not the same here!
         // but that is what you want
         that._current = value2;
    });

};

或者您可以让getValue方法执行回调，并将this设置为实例（使用call/apply）。

Answer 2

在外部作用域中声明一个变量来保存你的：

MyClass.prototype.myMethod1 = function(value) {
    var that = this;
    this.getValue("key", function(value2){
         that._current = value2;
    });

};

Answer 3

在

之前将其声明为变量

MyClass.prototype.myMethod1 = function(value) {
var oldThis = this;
 this.getValue("key", function(value2){
    /// oldThis is accessible here.
    });

};