[Vuejs]-How to save data in Vue instance

The problem is that this inside your ajax return function doesn’t refer to the vue instance anymore.

The solution is to save the this keyword into a variable inside the function. Example:

getUserAcc: function ( userID ) {
  var that = this;
  this.user = { _id : userID };
  $.ajax({
    url: "/listuser",
    type: "POST",
    data: this.user,
    success: function(data) {
      that.user = data;
      //Rest of your code
    },
    error: function(err) {
      console.log('error: ',err);
    },
  });
}

Here is more information about the behavior of the keyword this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this