global variable scope within a promise

I have this snippet of code in node.js, similar to javascript, well what happens is that it uses asynchronous methods to perform a validation of fields that is entered through a csv file, therefore I must analyze value by value of the fix _data , I also have in object user where it contains the property of user as global variable.
The problem is when I want to change the attributes of these properties within methods I get undefined error when I use this .

How should I use promise?

 this.user = {
                nombre: "",
                descripcion: "",
                email: "",
                fotografia: "",
            },

            isEmpty: function (str) {
                return (!str || 0 === str.length);
            },
            getUrlPath: function (fotografia) {
                return firebaseApp.database().ref("user").orderByChild("fotografia").equalTo(fotografia).on("child_added", function (snapshot) {
                    this.user.fotografia = snapshot.fotografia; **//error, undefined**
                        console.log(" su correo es " + snapshot.val().email);
                    // salida su correo es '[email protected]'
                });
            },

            //devuelve un correo desde firebase
            _getEmail: function (_email) {
                // var result = false;
                return firebaseApp.database().ref("user").orderByChild("email").equalTo(_email).on("child_added", function (snapshot) {
                    this.user.email = snapshot.email; **//error, undefined**
                        console.log(" su correo es " + snapshot.val().email);
                    // salida su correo es '[email protected]'
                });
                // return result;
            };
            validar: function (_data) {
                var partialErrors = "";
                var that = this;
                //fotografia
                this.getEmail(_data[3])).then(function (_valid) { **//aca funciona el this**
                      if (!_valid) {
                        partialErrors = partialErrors + "'" + _data[3] + "'" + " url no valida | ";

                    }

                    // nombre
                }).then(function (value) {
                    if (this._isEmpty(_data[1])) { **//error, undefined**                        
                        partialErrors = partialErrors + "'" + _data[2] + "'" + " valor incorrecto | ";
                    }
                })
            //descripcion
            .then(function (value) {
                if (this._isEmpty(_data[2])) { **//error, undefined**                        
                    partialErrors = partialErrors + "'" + _data[2] + "'" + " valor incorrecto | ";
                }
                //email
            }).then(function (value) {
                //obligue que ingresara aunque sea valida
                if (this._getEmail(_data[2])) { **//error, undefined**
                    console.log("valor :  " + this.user.email);  **//error, undefined**
                        partialErrors = partialErrors + "'" + _data[2] + "'" + " es incorrecto, sólo válido <[email protected]> | ";
                }
            }).catch(function (error) {
                console.log(error);
            });

            if (partialErrors.length > 0) {
                //error, undefined** No funciona el this
                this.errorFields.push("En " + _data + " se econtro errores en " + partialErrors);
                console.log(this.errorFields);
            }

            }