Add data to nested JSON object file

Ionic Framework ionic-v3
1

hi guys, i am trying to append some data to a nested JSON file but somehow it’s not getting there
Here is my JSON file
export class createusers{
mail: string
pass: string
field_title: {
und: string
}
field_firstname: {
und:[
{
value: string
}
]
}
field_lastname: {
und:[
{
value: string
}
]
}
field_date_of_birth: {
und:[
{
value: {
date: string
}
}
]
}
field_zip_code: {
und:[
{
value: string
}
]
}
// tid: string
field_city: {
und: string
}
field_phone_home: {
und:[
{
value: string
}
]
}
field_mobile: {
und:[
{
value: string
}
]
}
field_mac_add: {
und:[
{
value: string
}
]
}

}

and this is how i am trying to append the data in it

    this.createusers.mail=credentials.email;
    this.createusers.pass = credentials.password;
    this.createusers.field_title.und= credentials.title;
    this.createusers.field_firstname.und[0].value= credentials.firstname;
    this.createusers.field_lastname.und[0].value = credentials.lastname;
    this.createusers.field_date_of_birth.und[0].value.date = credentials.dob;
    this.createusers.field_zip_code.und[0].value = credentials.zip_code;
    this.createusers.field_city.und = credentials.tid;
    this.createusers.field_phone_home.und[0].value = credentials.home_number;
    this.createusers.field_mobile.und[0].value = credentials.mobile_number;
    this.createusers.field_mac_add.und[0].value = credentials.mac_add;

can anyone help me on that

2 
this.createusers.push({ 
something: yourValue, 
anotherSomething: yourSecondValue 
});
3

in the case of my nested JSON object, how do i access, for instance, date of birth?
do i put straight away date: credentials.dob?

4 
this.createusers.push({ 
something: yourValue, 
anotherSomething: yourSecondValue,
nestedSomething:  [{ nested: yourNestedValue, anotherNested: anotherNestedValue}]
});
5

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