Question

In the following customized class in javascript, in callback, why does this.obj have nothing but local variable obj has thing I want? Thanks.

function ClassTest(director) {
  this.obj = {"test1": "test1"};
}

function test1(input, callback) {
  callback("success");
}

ClassTest.prototype.test = function() {
  var obj = this.obj;
  test1("niuniu",function(e){
    console.log(this.obj);  // undefined
    console.log(obj);  // this one has stuff
    });
}

// run 
new ClassTest().test()

Answer 1

Because the function inside test1 is creating a new scope with different this context. Typical solutions are to bind or to cache this:

Binding:

test1("niuniu",function(e){
  console.log(this.obj);
}.bind(this));

Caching:

var self = this;
test1("niuniu",function(e){
  console.log(self.obj);
});