Add data to nested JSON object file

so according to what you said i have put it like that
this.createusers.mail=credentials.email;
this.createusers.pass = credentials.password;
this.createusers.field_title.und= credentials.title;
this.createusers.field_firstname.und.push({
value: credentials.firstname
});
this.createusers.field_lastname.und.push({
value: credentials.lastname
});
this.createusers.field_date_of_birth.und.push({
value:{
date:credentials.dob
}
});
this.createusers.field_zip_code.und.push({
value: credentials.zip_code
});
this.createusers.field_city.und = credentials.tid;
this.createusers.field_phone_home.und.push({
value: credentials.home_number
});
this.createusers.field_mobile.und.push({
value: credentials.mobile_number
});
this.createusers.field_mac_add.und.push({
value: credentials.mac_add
});

still not working