Ionic native facebook get information and use custom login api

I am using a ionic native facebook function to get the informations, I can get all infomations that I needed from facebook, but the problem I faced is that when I put the informations in my custom function to trigger login, the function of doServerFblogin not called.

Here is my code:

doFbLogin(){
  let permissions = new Array<string>();
  let nav = this.navCtrl;
  let env = this;
  //the permissions your facebook app needs from the user
  permissions = ["public_profile"];
  let fbStatus = '';

  this.fb.login(permissions)
  .then(function(response){
     let userId = response.authResponse.userID;
     let fbToken = response.authResponse.accessToken;
    fbStatus = response.status;
    let params = new Array<string>();

    //Getting name and gender properties
    env.fb.api("/me?fields=id,email,name,gender", params)
    .then(function(user) {
       user.picture = "https://graph.facebook.com/" + userId + "/picture?type=large";
       console.log("facebook login user :", user);
       console.log(user.picture);
       console.log(user.id);
       console.log(user.email);
       console.log(user.name);
       console.log(fbToken);       
       this.doServerFblogin(user.picture, user.id, user.email, fbToken, user.name); //here to call server api to login
    })
 }, function(error){
    console.log(error);
    });
}

Anyone know how to make it works. Thanks a lot.