How to access object data using variable value as element in vue js

Question

How i can access the object data using variable value as key?

Let say i have this object:

let item = {
    name: {
        first_name: "Mark",
        last_name: "James"
    }
}

How i can get the value of first name using variable value?
I know i can access it using

item.name.first_name

What if i want to access it using variable value like this:

let key = "name.first_name"

console.log(item[key])