Step: Object Identifier-Index: .foo

The simplest useful filter is $.foo. It produces the value at the key "foo", or empty list if there's none present.

For example given input {"make": "Nissan", "model": "Kicks"} and filter $.make it produces "Nissan".

Now you try to create a JQ expression that would product the result.

{
  "firstName": "Charles",
  "lastName": "Doe"
}

[
  "Charles"
]