How to get the customer's data on the frontend checkout page's client side (with JavaScript) using the «window.customerData» object?

Context: How to get the customer’s data on the frontend checkout page’s client side (with JavaScript)?

window.customerData

A registered customer

A guest customer (a «guest checkout»)

How is it implemented?

Details: How is the frontend checkout configuration passed to the client side (JavaScript)?

See also:

• How to get the customer’s data on the frontend checkout page’s client side (with JavaScript)?
• frontend-checkout-customer-data - tagged articles.
• The difference between window.customerData, Magento_Customer/js/customer-data, and Magento_Customer/js/model/customer on a frontend checkout page.